i think what the OP is trying to say
if Mastercard usually had a 2% fee. and somehow someone hacked a retail terminal to charge 300% fee
EG $6.70 transaction with a $15.70 transaction fee.
comparable to
6.7btc tx out with 15.7btc fee
https://blockchain.info/tx/6fe69404e6c12b25b60fcd56cc6dc9fb169b24608943def6dbe1eb0a9388ed08now knowing the retailer cant amend this issue as they never received the funds and the funds are alreadygone and its visa that received the fee (bitcoin mining pool received)
there shouldnt just be something at the retailer(wallet) side that prevents over spends. there should be something deeper at the pool/network level that realises that something has gone wrong.
though i still oppose the dire need of transaction fee's to suppliment the block reward(for atleast a decade), those that do want to pay a fee should still be protected from making mistakes. EG. if fee is $1+(0.0025+), then nodes shouldnt relay it. same for miners if fee is $1 (>0.0025) then dont add to block.
that way people can still pay 4cents. or if stupid upto $1 in a race for priority.. but anything more is an obvious mistake especially a $6000 fee.
I understand but what happens if the fee is not a mistake? It should not be the responsibility of the base protocol to assess what is and isn't a valid fee. Any sort of protection of the end user should be handled by the wallet, and most importantly by the user. That means check what you're sending. It could be as simple as adding an extra confirmation step for fees over a certain amount, say 10x the current estimated fee. But this would be handled by the wallet application.
Your scenario relies on a hacker getting access to a payment terminal. That's an extreme case and I would hope whoever is responsible for the terminal to accept responsibility and come to some sort of resolution. At this point the core Bitcoin software is not really suited for day to day retail transactions. But many other wallets/services are. Most retailers rely on services like Bitpay, which are similar to Visa or Mastercard in the fiat world in that they act as a third party to process the transaction, taking on some responsibility. When something goes wrong, i.e. hacked terminal, Visa and Mastercard will often foot the bill. I would expect the same from Bitpay or any of its competitors.