Ok, I think it's time for me to say that if you put a price on something without it being an auction, you should definitely stick to that price. Otherwise you are leaving yourself open to attack. Make your word mean something. People can respect you more for doing so.
Here's a tip: Just remember, you still have to ship it. There's nothing that says that you can't ask for slightly more than the cost of postage to ship it to the individual. I usually ship in soft sleeve, hard cover and bubble envelope. They don't come free. However, remember that too high a shipping cost can be grounds for people backing out of deals. If I were you, I'd negotiate a shipping cost. That's my 2 cents, and it's only a guideline. What you do is up to you. Hope that helps.