I've got to say that I like Kathleen's comment. Strong, direct, and still communicating that you want to continue a business relationship with the customer. My only additional recommendation would be to go over it in your head a number of times or possibly role-play it with a friend. Get used to the words coming out firmly but with a smile.
I hope it goes well. Let us know how it works out, I'm curious. Keep up the good work.