My mission is to never rip anyone off even if I can. To be fair and equitable so that we both come out ahead. This is probably why I'm not rolling in dough like a lot of other carpenter/contractors.....
If I can't remember if I charged a client for something, I generally will err in their favour as opposed to risking charging them twice. I recently had a customer accuse me of charging them for materials that they bought and I just about lost it on them. Considering we'd gone over the invoice 3 times previously and I spelled out exactly what I quoted, what I paid for, and what they paid for. (I quoted on a bathroom reno and they decided halfway through to replace the light fixture, medicine cabinet and towel bar etc so I said "just buy the items and I'll install them". Then he came back and said that he bought the toilet, the tiles, grout, mortar, vanity, etc etc etc when I had receipts showing I actually paid for them. I called a meeting and laid out everything for him yet again and THIS time, I had him sign a paper stating that I had gone over everything and should he, or an agent of his, contact me a 5th time on this subject a charge of $500.00 will become due to cover off my time and materials to produce yet another statement of charges.