>Do you know if most common bitcoin wallets and payment processing APIs accept or reject payments which have opted in to RBF? It might be a configuration issue, but how big a deal this is depends entirely on what the default configuration is.
I don't know. Any serious business (which, unfortunately, doesn't describe many bitcoin ones) should be monitoring them, and frankly, as I said above, if you're accepting 0-confs and not checking for the flag, and sending irreversible products, you're beyond my sympathy. Official guides to accepting bitcoin always say to wait several confirmations, and anyone accepting zero confs has make that decision deliberately.
> If I want to accept 0-conf transactions for say, my Bitcoin-powered public coffee machine (not an actual product, but should be...), then I feel a bit safer if canceling 0-conf payments is hard to do and unreliable, even if I don't have any strong guarantees.
As far as I can see, this is entirely alleviated by the opt-in part. Just reject any transactions that opt in.
I've seen people complain that users will opt in by default and be turned off if they need to wait, but that's a problem with whatever wallet they're using. If I was designing a wallet, I'd make an "enable undo?" button, with a warning that said "if this is checked, you can undo until it posts, which usually takes a few minutes. Be aware that some merchants won't accept such transactions, so if you're buying something instantly then make sure the merchant is ok with it.", and merchants should have signs telling people to turn off the undo feature.