How to deal with people that won't leave your work?

I work in a store that accepts many payments for utilities, phones, loan payments, etc. We just take the payment and send it off via Western Union for the customer. After a payment is processed a control number is printed on the receipt. Sometimes the company asks the customer to call in with this number. There is this one man who pays two utility bills every month, and after he does he stands here, taking up my counter space, calling these companies (who usually put him on hold for 5-10 minutes) so he can confirm his payment. He also calls other companies when he's done to make payments over the phone with his card, payments that my business doesn't accept. I need to get work done that I can't do with customers in the store, and he takes up a lot of my time. How can I tell him to leave without sounding like I'm in the wrong?