<PAGE>

          CERTAIN PORTIONS OF THIS EXHIBIT HAVE BEEN OMITTED AND FILED
         SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION PURSUANT
                     TO A REQUEST FOR CONFIDENTIAL TREATMENT

                                                                   Exhibit 10.28

                          Supplemental Agreement No. 6

                                       to

                           Purchase Agreement No. 2191

                                     between

                               The Boeing Company

                                       and

                            COPA Holdings, S.A., Inc.

                      Relating to Boeing Model 737 Aircraft

            THIS SUPPLEMENTAL AGREEMENT entered into as of September 9, 2004, by
and between THE BOEING COMPANY, a Delaware corporation with its principal office
in Seattle, Washington, (Boeing) and COPA HOLDINGS, S.A., INC. (Customer);

            WHEREAS, the parties hereto entered into Purchase Agreement No. 2191
dated November 25, 1998 (the Agreement), as amended and supplemented, relating
to Boeing Model 737-700 and 737-800 aircraft (the Aircraft); and

            WHEREAS, Boeing and Customer have mutually agreed to the exercise of
two (2) 737-800 Purchase Right Aircraft both of which have a scheduled delivery
month of June 2006; and

            WHEREAS, Boeing and Buyer have mutually agreed to amend the
Agreement to incorporate the effect of these and certain other changes;

NOW THEREFORE, in consideration of the mutual covenants herein contained, the
parties agree to amend the Agreement as follows:

1.    Table of Contents, Tables and Exhibits:

      1.1.  Remove and replace, in its entirety the "Table of Contents", with
            the "Table of Contents" attached hereto and hereby made a part of
            the Agreement, to reflect the changes made by this Supplemental
            Agreement.

      1.2.  Attached hereto and hereby made a part of the Agreement is Table 1-3
            entitled "Aircraft Information Table for Model 737-7V3 Aircraft"
            which reflects the addition of two (2) 737-7V3 Aircraft both of
            which have a scheduled delivery month of June 2006.

                                     Page 1
<PAGE>

2.    Letter Agreements: Remove and replace Letter Agreement No. 6-1162-LAJ-874
      entitled "**Material Redacted**" with Letter Agreement No.
      6-1162-LAJ-874R1 entitled "**Material Redacted**" attached hereto and
      hereby made a part of the Agreement.

The Purchase Agreement will be deemed to be supplemented to the extent herein
provided as of the date hereof and as so supplemented will continue in full
force and effect.

                                     Page 2
<PAGE>

                                TABLE OF CONTENTS

<TABLE>
<CAPTION>
                                                                           SA
ARTICLES                                                                  NUMBER
----------------------------------------------------------------          ------
<S>                                                                       <C>
     1.  Quantity, Model and Description                                   SA 3
     2.  Delivery Schedule
     3.  Price
     4.  Payment                                                           SA 3
     5.  Miscellaneous

TABLE

    1-1  Aircraft Information Table for Model 737-7V3 Aircraft             SA 4
    1-2  Aircraft Information Table for Model 737-8V3 Aircraft             SA 5
    1-3  Aircraft Information Table for Model 737-8V3 Aircraft             SA 6

EXHIBIT

    A-1  Aircraft Configuration for Model 737-7V3 Aircraft                 SA 3
    A-2  Aircraft Configuration for Model 737-8V3 Aircraft                 SA 3

     B.  Aircraft Delivery Requirements and Responsibilities               SA 3

SUPPLEMENTAL EXHIBITS

  BFE1.  BFE Variables                                                     SA 3
   CS1.  Customer Support Variables                                        SA 3
   EE1.  Engine Escalation/Engine Warranty and Patent Indemnity
  SLP1.  Service Life Policy Components

LETTER AGREEMENTS

 2191-01 Demonstration Flight Waiver
 2191-02 **Material Redacted**
 2191-03 Seller Purchased Equipment

RESTRICTED LETTER AGREEMENTS

 6-1162-DAN-0123        Performance Guarantees
 6-1162-DAN-0124        **Material Redacted**
 6-1162-DAN-0155        **Material Redacted**
 6-1162-DAN-0156        Year 2000 Ready Software, Hardware and Firmware
 6-1162-DAN-0157        Miscellaneous Matters
 6-1162-MJB-0017        **Material Redacted**
 6-1162-MJB-0030        **Material Redacted**
</TABLE>

                                       i
<PAGE>

<TABLE>
<S>                     <C>
6-1162-LAJ-874          **Material Redacted**
6-1162-LAJ-8741         **Material Redacted**
</TABLE>

<TABLE>
<CAPTION>
  SUPPLEMENTAL AGREEMENTS         DATED AS OF:
---------------------------    -----------------
<S>                            <C>
Supplemental Agreement No. 1     June 29, 2001
Supplemental Agreement No. 2   December 21, 2001
Supplemental Agreement No. 3     June 14, 2002
Supplemental Agreement No. 4   December 20, 2002
Supplemental Agreement No. 5   October 31, 2003
Supplemental Agreement No. 6   September 9, 2004
</TABLE>

                                       ii
<PAGE>

                                    TABLE 1-3
              AIRCRAFT INFORMATION TABLE FOR MODEL 737-7V3 AIRCRAFT
           AIRCRAFT DELIVERY, DESCRIPTION, PRICE AND ADVANCE PAYMENTS

<TABLE>
<S>                    <C>                    <C>        <C>          <C>                        <C>                          <C>
AIRFRAME MODEL/MTGW:   737-700                154,500                 DETAIL SPECIFICATION:      D6-38808-42-1-D (10/5/2002)

ENGINE MODEL:          CFM56-7B22                                     AIRFRAME PRICE BASE YEAR:  Jul-00

AIRFRAME PRICE:                                          $41,058,000

OPTIONAL FEATURES:                                       $ 3,011,200  AIRFRAME ESCALATION DATA:
                                                         -----------
SUB-TOTAL OF AIRFRAME AND FEATURES:                      $44,069,200  BASE YEAR INDEX (ECI):                                  145.40

ENGINE PRICE (PER AIRCRAFT):                             $         0  BASE YEAR INDEX (ICI):                                  130.30

AIRCRAFT BASIC PRICE (EXCLUDING BFE/SPE):                $44,069,200
                                                         ===========

BUYER FURNISHED EQUIPMENT (BFE) ESTIMATE:                $ 1,000,000

SELLER PURCHASED EQUIPMENT (SPE) ESTIMATE AT DELIVERY:   $ 1,700,000

REFUNDABLE DEPOSIT PER AIRCRAFT AT PROPOSAL ACCEPTANCE:  $    75,000
</TABLE>

<TABLE>
<CAPTION>
                                                      ESCALATION    ADVANCE PAYMENT PER AIRCRAFT (AMTS. DUE/MOS. PRIOR TO DELIVERY):
                          ESCALATION                 ESTIMATE ADV   ----------------------------------------------------------------
               NUMBER OF    FACTOR    MANUFACTURER   PAYMENT BASE     AT SIGNING     24 MOS.      21/18/12/9/6 MOS.      TOTAL
DELIVERY DATE  AIRCRAFT   (AIRFRAME)  SERIAL NUMBER  PRICE PER A/P         1%           4%              5%                30%
-------------  ---------  ----------  -------------  -------------    ----------   ----------     -----------------   -----------
<S>   <C>         <C>       <C>          <C>         <C>               <C>         <C>               <C>              <C>
  Jun-2006        1         1.2299       34535       $  55,901,000     $484,010    $2,236,040        $2,795,050       $16,770,300
  Jun-2006        1         1.2299       34536       $  55,901,000     $484,010    $2,236,040        $2,795,050       $16,770,300
</TABLE>

<PAGE>

                          ATTACHMENT C2 (MODEL 737-800)
                               TO 6-1162-LAJ-874R1

6-1162-LAJ-874R1

COPA HOLDINGS, S.A.
Apartado 1572
Avenida Justo Arosemena y Calle 39
Panama 1, Panama

**Material Redacted**

Reference:  Purchase Agreement No. 2191 (the Purchase Agreement) between The
            Boeing Company (Boeing) and COPA HOLDINGS, S.A. (Customer) relating
            to Model 737 aircraft (the Aircraft)

This Letter Agreement amends and supplements the Purchase Agreement. This Letter
Agreement supercedes and replaces in its entirety Letter Agreement
6-1162-LAJ-874. All terms used but not defined in this Letter Agreement have the
same meaning as in the Purchase Agreement. In consideration of the Aircraft
orders, Boeing provides the following to Customer.

1     **Material Redacted**

2.    **Material Redacted**

3.    **Material Redacted**

4.    **Material Redacted**

5.    **Material Redacted**

6.    **Material Redacted**

7.    **Material Redacted**

8.    Confidentiality. Customer understands that certain commercial and
      financial information contained in this Letter Agreement are considered by
      Boeing as confidential. Customer agrees that it will treat this Letter
      Agreement and the information contained herein as confidential and will
      not, without the prior written consent of Boeing, disclose this Letter
      Agreement or any information contained herein to any other person or
      entity.

ACCEPTED AND AGREED TO this

Date: September 9, , 2003

THE BOEING COMPANY

COPA HOLDINGS, S.A.

<PAGE>

                          ATTACHMENT C1 (MODEL 737-700)
                               TO 6-1162-LAJ-874R1

                                   **2 pages**

<PAGE>

                          ATTACHMENT C2 (MODEL 737-800)
                               TO 6-1162-LAJ-874R1

                              **Material Redacted**

                                   **2 pages**