#1 Once you have their money, never give it back.
I have actually used this rule in real life. I work at a library, and people come in to print things off the computers, at a dime a page. In the past, it was sort of an honor system, and people would print pages of stuff and then go through the stack and pick out only what they wanted to keep and leave the rest. We switched to a new system where they have to pay us before we print the pages. Some of the staff had some trouble adapting to the new system and would refund money for pages people didn't want. I actually quoted them Rule #1, with attribution (they already know I'm a geek).