<PAGE>
 
                                                                   Exhibit 10.13









                      IRIDIUM SERVICE PROVIDER AGREEMENT
                                  EXHIBIT E:

                        ADDITIONAL TERMS AND CONDITIONS

<PAGE>
 
                                   Exhibit E
                        Additional Terms and Conditions

        The following terms and conditions are made part of the Iridium Service 
Provider Agreement between INA and SkySite Communications Corporation to which 
                                   ----------------------------------
it is appended.  When this Exhibit is signed by both parties, the terms hereof 
shall supersede and control over any provisions of the Iridium Service Provider
Agreement that are inconsistent herewith.

        A.  Ethical Responsibilities of the Parties: INA and SkySite shall each
            refrain from doing anything that would tend to discredit, dishonor,
            reflect adversely upon, or in any manner injure the reputation of
            the other or adversely affect the other, except that a party's
            enforcement of its rights and performance of its duties and
            obligation contained in this Agreement shall not be deemed a
            violation of the provisions of this paragraph. Each party shall be
            governed in all its dealings under this Agreement by the highest
            standards of honesty, integrity and fair dealing. Each party shall
            comply with all applicable federal, state and local laws ordinances
            and regulations in performing its duties and obligations contained
            in this Agreement.

        B.  Notwithstanding the provisions of Section 7, the initial term hereof
            will continue for a period of three (3) years after execution of
            this Agreement.

        C.  This Agreement may be cancelled by SkySite Communications
            Corporation, without further obligation except for those obligations
            which are outlined in Article 16.B1, either (a) without cause, by
            providing written notice to INA, on or before February 1, 1998; or
            (b) if the Commencement Date does not occur before November 23,
            1998.

IRIDIUM U.S., L.P.                      SERVICE PROVIDER

Keith W. Bubb                           Philip A. Kernan, Jr.
-------------------------------------   -------------------------
By:                                     By:

/s/ Keith W. Bubb                       /s/ Philip A. Kernan, Jr.
-------------------------------------   -------------------------
Signature:                              Signature:

Director, Gateway Business Operations   President and CFO
-------------------------------------   -------------------------
Title:                                  Title:



<PAGE>
 
                                 ADDENDUM TWO
                  SATELLITE MINUTES COMMITMENT PLAN ADDENDUM

        This Satellite Minutes Commitment Plan (the "Plan") is dated as of 
10 August, 1998 and is by and between Iridium U.S., L.P., d/b/a Iridium North 
America ("INA") and Project 77, Inc. ("Service Provider").

                                   RECITALS

        Whereas INA and Service Provider have entered into a service provider 
agreement, dated November, 1997 (the "Service Provider Agreement") to provide 
Iridium satellite services and the parties now desire to modify the pricing 
structure set forth therein;

        Whereas, Service Provider desires to obtain favorable pricing rates 
based upon outbound minutes of domestic and international Iridium homed 
satellite use purchased and paid for by Service Provider ("Satellite Minutes") 
specifically excluding any promotional or other free minutes of use and INA 
desires to obtain a firm commitment from Service Provider for its Satellite 
Outbound Minutes over a fifteenth month period; and

        Whereas, INA and Service Provider desire to enter in to its Plan upon 
the terms and conditions set forth herein.

                                   AGREEMENT

        Not, therefore, in consideration of the mutual agreements and 
understandings herein contained, the parties hereto agree as follows:

1.      COMMITMENT PLAN. INA is offering a volume pricing plan based upon
        ---------------
        Service Provider's level of commitment. The purpose of this Plan is to
        grant favorable pricing to Service Provider for cultivating additional
        Satellite Minutes through increased sales and Iridium subscribers. In
        addition, this Plan is designed to hold Service Provider accountable to
        its commitment and discouraging Service Provider from making unrealistic
        or impractical commitments, thereby gaining a market pricing advantage
        relative to other service providers.

2.      COMMITMENT. During the term of this Plan, Service Provider hereby agrees
        ----------
        to commit to the (Initial One of the following):

        _____ Entry Level Program and a minimum gross revenue of $500,000.00.
        
        _____ Second Level Program and a minimum of 1,000,000 Satellite Minutes.

        _PAK_ Third Level Program and a minimum of 3,000,000 Satellite Minutes.

Service Provider shall be entitled to the pricing and rebate structure and 
obligated to the assessment set forth below associated with the program selected
above.


<PAGE>
 
Entry Level Program.
-------------------

        Pricing.  Service Provider shall be charged the standard rates for 
Satellite Minutes (the "Standard Rate") which are contained in Exhibit B to the
                                                               --------- 
Service Provider Agreement.

        Commitment.  Service Provider shall guaranty INA minimum gross revenue 
from Satellite Minutes equal to $500,000.00 during the term of this Plan.

        Assessment.  If Service Provider fails to purchase at least $500,000.00 
of Satellite Minutes, Service Provider shall pay to INA the difference between
the gross revenue received from Service Provider for Satellite Minutes and 
$500,000.00 thirty days after the end of the term of this Plan.

        Termination.  If Service Provider fails to purchase at least 
$500,000.00 of Satellite Minutes, INA shall have the option of terminating the
Service Provider Agreement, in its sole and absolute discretion.

Secondary Level Program.
-----------------------

        Pricing.  Service Provider shall be charged the Standard Rate for 
Satellite Minutes less a five percent (5%) discount. Once Service Provider 
reaches the first place breakpoint by purchasing 2,000,000 Satellite Minutes 
during the term of this Plan, INA shall increase the applicable discount off of 
the Standard Rate to seven percent (7%) for Satellite Minutes purchased 
thereafter. Once Service Provider reaches the second price breakpoint by 
purchasing 3,000,000 Satellite Minutes during the term of this Plan, INA shall 
increase the applicable discount off of the Standard Rate to ten percent (10%) 
for Satellite Minutes purchased thereafter.

        Commitment.  Service Provider agrees to purchase a minimum of 1,000,000 
Satellite Minutes during the term of this Plan.

        Rebates.  If Service Provider reaches the first price breakpoint by 
purchasing 2,000,000 Satellite Minutes during the term of this Plan, INA shall 
credit Service Provider with a rebate. The rebate shall be calculated by taking 
two percent (2%) of Service Provider's Blended Rate (defined below) and 
multiplying the product thereof by one million (1,000,000). If Service Provider 
purchases 3,000,000 Satellite Minutes during the term of this Plan, INA shall 
credit Service Provider with a second rebate. The second rebate shall be 
calculated by taking three percent (3%) of Service Provider's Blended Rate and 
multiplying the product thereof by one million (1,000,000).

        Assessment.  If Service Provider's Satellite Minutes are less than 
1,000,000 at the end of term of this Plan, Service Provider shall pay INA an 
assessment for each unused Satellite Minute within thirty days from the end of 
this Plan. The assessment shall be equal to the difference between 1,000,000 and
Service Provider's Satellite Minutes multiplied by $1.00. For example, assume 
a Service Provider's Satellite Minutes are 800,000 at the end of this Plan. In 
this case the difference between the Service Provider's Satellite Minutes and 
its commitment to is 200,000. Therefore, the Service Provider's assessment will 
be equal to $200,000.00 ([1,000,000 - 800,000] * $1.00).

<PAGE>
 
Third Level Program
-------------------

        Pricing. Service Provider shall be charged the Standard Rates for 
Satellite Minutes less a ten percent (10%) discount. Once Service Provider 
reaches the first price breakpoint by purchasing 4,000,000 Satellite Minutes 
during the term of this Plan, INA shall increase the applicable discount off of 
the Standard Rate to twelve percent (12%) for Satellite Minutes purchased 
thereafter. Once Service Provider reaches the second price breakpoint by 
purchasing 5,000,000 Satellite Minutes during the term of this Plan, INA shall 
increase the applicable discount off of the Standard Rate to fifteen (15%) for 
Satellite Minutes purchased thereafter.

        Commitment. Service Provider agrees to purchase a minimum of 3,000,000 
Satellite Minutes during the term of this Plan.

        Rebate. If Service Provider reaches the first price breakpoint by 
purchasing 4,000,000 Satellite Minutes during the term of this Plan, INA shall 
credit to Service Provider a rebate. The rebate shall be calculated by taking 
two percent (2%) of Service Provider's Blended Rate (defined below) and 
multiplying the product thereof by one million (1,000,000). If Service Provider 
reaches the second price breakpoint by purchasing 5,000,000 Satellite Minutes 
during the term of this Plan, INA shall credit to Service Provider a second 
rebate. The second rebate shall be calculated by taking three percent (3%) of 
Service Provider's Blended Rate and multiplying the product thereof by one 
million (1,000,000).

        Assessment. If Service Provider's Satellite Minutes are less than
3,000,000 at the end of the term of this Plan, Service Provider shall pay INA an
assessment for each unused Satellite Minute thirty days from the end of this
Plan. The assessment shall be equal to the difference between 3,000,000 and
Service Provider's Satellite Minutes multiplied by $1.00.

3.      EVALUATION.  INA shall evaluate Service Provider's performance six 
        ----------
        months from the Commencement Date and thereafter on a quarterly basis.
        If, in INA's sole and absolute discretion, INA determines that Service
        Provider will not be able to meet its commitment obligations contained
        herein, INA reserves the right to reduce or eliminate the pricing
        discount granted to Service Provider to recover the assessment described
        above. INA's decision not to modify the discount applied shall not
        effect Service Provider's commitment or the assessment due for failure
        to meet its commitment.

4.      REBATE CALCULATION. Service Provider's Blended Rate shall be equal to
        ------------------
        the weighted average of the Standard Rate for Satellite Minutes during
        the applicable period. The "applicable period" shall mean the period
        between the billing cycle immediately following Service Provider's
        Satellite Minutes exceeding the prior price breakpoint and the billing
        cycle immediately following Service Provider's Satellite Minutes
        exceeding the current price breakpoint. Service Provider shall, within
        thirty (30) days of a written request therefor, reimburse INA for full
        amount of any rebate credited hereunder which should not have been
        credited to Service Provider as the result of any re-calculation, for
        any reason, of the total amount of Service Provider's Satellite Minutes:
        INA shall apply a rebate or reimbursement of a rebate to Service
        Provider's invoice for the billing cycle following of the close of the
        billing cycle in which the commitment is exceeded or the recalculation
        is made.

5.      TERM. This Plan shall be effective for fifteen (15) months from the
        ----
        Commencement Date or December 31, 1999 whichever is later. If the
        Service Provider Agreement is

<PAGE>
 
        terminated prior to such date, this Plan shall terminate and Service
        Provider shall pay to INA the assessment calculated above using Service 
        Provider's actual performance through the billing cycle
        immediately following the termination date and a minimum forecast
        analysis prepared by INA, in its sole and absolute discretion, for such
        time period. The assessment shall be paid thirty (30) days after the end
        of the billing cycle following the termination date.

6.      REVISIONS.  Upon thirty days written notice to Service Provider, INA
        ---------
        reserves the right to revise the terms of this Plan in its sole and
        absolute discretion depending on market conditions during the term of
        this Plan. If there is a cumulative pricing increase of five percent
        (5%) or more during any six month period, Service Provider shall have
        the option to revise its commitment election; provided, however, Service
        Provider shall not be entitled to revise its election if it could not
        reasonably expect to meet its commitment had the pricing increase not
        occurred.

7.      AMENDMENT. Unless otherwise modified herein, the terms and conditions
        ---------
        set forth in the Service Provider Agreement shall remain in full force
        and effect. All capitalized terms not defined herein shall have the
        meaning assigned in the Service Provider Agreement.


Dated effective as of the date first written above.

Iridium U.S., L.P.                   Project 77, Inc.
                                     
                                     
By: /s/ Jim Walz                     By: /s/ Philip A. Kernan, Jr.      
   ----------------------               --------------------------   
   Jim Walz                             Philip A. Kernan, Jr.
                                        
Title: President                     Title: President                





<PAGE>
 
                           AMENDMENT TO ADDENDUM TWO
                  SATELLITE MINUTES COMMITMENT PLAN ADDENDUM

        The following terms and conditions are made a part of Addendum Two to 
the Iridium Service Provider Agreement between INA and Service Provider, to 
which is appended. When this Amendment is signed by both parties, the terms 
hereof shall supersede and control over any provisions of Addendum Two that are 
inconsistent herewith.

A.  Section 3, EVALUATION, Section 3 is deleted in its entirety and replaced 
               ---------- 
with the following:


"3. EVALUATION. INA shall evaluate Service Provider's performance six months 
 -------------
from the Commencement Date and thereafter on a quarterly basis. The initial six
month's performance review will be undertaken for all service providers as a
group, and if a substantial majority of service providers have been unable to
meet their program commitment levels, INA, at its sole and absolute discretion,
may make reasonable and non-discriminatory adjustments to the rates and
commitment levels applicable to service providers, including Service Provider. A
decision by INA not to modify the Program Level applied shall leave in effect
Service Provider's commitment and the assessment due to failure to meet its
commitment.

If during the initial six month's performance review, in INA's sole and absolute
discretion INA determines that a general adjustment of rates and commitment
levels is unnecessary but that Service Provider has not reached 300,000
Satellite Minutes, INA shall adjust Service Provider's commitment to a lower
level Program hereunder. INA shall have sole and absolute discretion as to which
lower level Program hereunder Service Provider's commitment will be adjusted. In
the case of such Program adjustment, Service Provider shall reimburse, as set
forth in Section 4 below, INA five percent (5%) of all Service Provider's actual
charges incurred under this Agreement as of the date of such Program adjustment
("Recapture of Discount")."

B.  Section 4, REBATE CALCULATION. Section 4 is amended to add the following:
               ------------------         
"The process provided herein shall apply to Rental Service Provider's payment of
the Recapture of Discount calculated pursuant to section 3."



<PAGE>
 
Unless otherwise modified herein, all other terms and conditions contained in 
Addendum Two to the Iridium Service Provider Agreement shall have full force 
and effect.


Dated August 18, 1998
      ---------------

Iridium U.S., L.P.                   Project 77, Inc.
                                     
                                     
By: /s/ Jim Walz                     By: /s/ Philip A. Kernan, Jr.      
   ----------------------               --------------------------   
   Jim Walz                             Philip A. Kernan, Jr.
                                        
Title: President                     Title: President