I guess, but you might as well keep a formal invoice if you're going to try and fight digital goods. I guess you can put burden of proof on them by having to actively input the code to get their coins, or take their BTC address and pray to God that they would use the same one they gave you, but I'm not sure how that would hold up in a chargeback investigation without someone weaseling their way out of something. Is there a Instawallet fork that you can email/password protect, sort of like a burner account at an exchange, or at least have some sort of two-factor between the URL itself and another password?
That's the ticket. Gonna do some asking around, been doing a lot of that lately, see what I can get away with.
Could also mail them a physical Yubikey and have that be signed for, but that sort of takes the instant advantage of Credit Cards out of it. Convience is still there, as is security for buyer and seller. Hurm.... maybe for >$100 worth of BTC, it has to be authenticated that way?