We favor the open ended 30 day written notice that each party could exercise. Be aware of costs that may go along with an out clause and be sure you will be able to obtain all of your receivable data in the event of a termination. This area should state how your fee is calculated. It should say when you will be invoiced and the terms of that invoice. If the company charges interest, that should be stated as well as the percentage and how it is calculated.
Some agreements have an additional provisions clause.
Additional provisions would be anything over and above the normal billing that you may require or that you negotiated separately from the billing fee. Some of these things are considered to be mutually beneficial by the billing service and may be no cost to a client. If this is true it should be listed and state no cost. Those items that have an additional cost should also be listed and the fee should be listed as well. This information up front will eliminate any unexpected surprises later on.