getting back overpayment from Supplier

Hi all,
One of our suppliers was overpaid. We have been trying to get back this amount but for the last week none of my calls have been returned. I was wondering what options are available to get back this money.

A polite letter outlining the problem and emphasizing the good working relationship you have had to date and your hopes that this can continue into the future if they help you resolving this current difficulty?

Two other possibilities come to my mind:
(1) Can you create an 'incentive' for this supplier, by giving them a large order in the future and then creating a balance between the two transactions?
(2) If you don't plan to spend with them in the future, then put a 'junior' on the task of calling them up regularly, and after a few weeks if this doesn't work, maybe employ a credit collection agency.