While no one wants to go through the formal bidding process, it is still a good idea to have a letter of agreement in place and to monitor the terms and conditions. Have a fixed period of time that the agreement covers. Some agents charge a penalty for single orders. One Correction In Billing per title per year is enough. Some agents charge extra for GPO or Neodata.
Some vendors have the computer calculate the service charge and a target margin is maintained.
Any change in mix of titles or decreases in publisher discount will result in an automatic service charge increase. Check out the service charge on credits. All of the above issues can be incorporated into a basic service agreement. Too often the agents and librarians have been operating on a good faith relationship and over time we all lose sight of the original agreement. It is good business to have a clear understanding in writing of the requirements and have the service charges documentated.