Pawsplay,
I don't know which web site your purchasing through, but didn't you get a reciept? RPGNow (and I assume Drivethrurpg.com) issues a reciept for purchases. It it was ever a problem I'd just print the screen.
Kinko's - the times I've used them to print, it was at the rent-a-PC, where no one asked anything. I just went to the machine, stuck my credit card in, loaded my USB drive and printed what I needed. Have you considered doing this? I typically only use the counter folks to bind print jobs, and they've never given me a problem.
I think the copy center people have problems when you are asking THEM to do the printing, which could make them accessories to copyright violations. I think if you used the self-serve copying machines, where there are signs warning YOU against copyright violations, you should be fine.
Side question - Has this problem ever happened to anyone buying from Paizo? The copies I have always have my name and e-mail watermarked on the bottom, but I've never been asked.