How do I make this guy pay me for my design and remain professional?

I designed a tattoo for a friend and he said he'd pay me for it. After spending a good 4-5 hours working on the piece, I told him it was ready to be picked up. That was 4 days ago. I haven't done commission work before this, so I didn't have the hindsight to draw up a contract or ask for the money up front. How do I tell him he needs to come get his design and pay me for it while being both persuasive and professional? I want him to know that I feel like it's completely rude for him to ask me to design something, have me spend good deal of time on it, and then never follow through. I can usually articulate things like this pretty well, but I feel like I might start with a professional email and end with a nasty one.