Key is that you need to be willing to give that $3 back at any time, for any reason. An apology and a refund will shut up almost everyone, even if you really did screw it up; or, at least, that has been my experience, and as some HN users can attest, I do screw it up from time to time.
I think a vendor fighting someone who is disputing a $3 charge is the height of irrationality. It's going to cost you way more than $3 to just deal with the dispute, so even if you "win" you lose, and worse, then the customer is angry and might get you a lot of bad publicity.
I think a vendor fighting someone who is disputing a $3 charge is the height of irrationality. It's going to cost you way more than $3 to just deal with the dispute, so even if you "win" you lose, and worse, then the customer is angry and might get you a lot of bad publicity.