You simply put it as you did hear. Your priority is with your direct boss. You are sorry that you can no longer provide this to the staff, but your time is now directed elsewhere. Simple. They should understand.

Is your direct boss also over these supervisors, or on the same level as these supervisors? If your boss is over them, I would suggest talking to your boss about this situation and ask that he/she do a memo letting them know that they will have to go back to doing it themselves due to the reasons you stated. If he/she is on the same level as them, maybe he/she could bring it up in the next meeting and find an agreeable solution. They may not mind taking it back and hopefully appreciated the times you were able to handle it for them.

Our agency did the same thing. We put together an expense packet for individuals to fill out along with procedures and an example on how to fill out the expense statement. A short meeting was held, with my boss'es approval and the staff have been doing it themselves ever since.

I agree with M Clark, couldn't have said it any differently. If that is not the case for yourself the way you posed the questions sounded just fine and I would just let them know.

First, develop a solution which should include a packet of information with streamlined procedures on how expense reporting needs to be completed.

Second, have your boss validate this new procedure with an email to the staff with the meeting date/time. If it comes from the boss, it is likely to be taken more seriously. Unfortunate, but true!

Third, hold a meeting with a powerpoint presentation showing them how it's done and give them the packet of info to go for future reference.

Good luck!