If you were overpaid, you will receive a written notice in the mail. If the request is a duplicate payment, copies of both receipts are required.• Proof of exemption (if applicable). This brief but important letter notifies a customer that he or she has overpaid you. It can serve as a very good customer relations letter. We may mail you information and resources to make it easier for you to file, pay, and perform other business with the Tax Department. What forms of payment do you accept? For the open invoice in the Payment column, enter how much of the credit you want to apply. To make a payment or apply for a payment plan, visit the online payment portal today through UIS. Students can login to their personal UIS account.