Quite understand, and sympathise.I had that exact experience except mine was at the checkout of a French supermarket - very, very embarassing.
Which is why we always go in with at least 4 cards and a load of cash.
What has bothered us more than once is going to a petrol station where you pay at the pump and all our cards being declined.
Got very dodgy one time, we only just managed to struggle into an old style one where you paid at the desk. There the blokey had been ripped off so often he made you pay first and then fill up, and he controlled the pump from where he was behind the desk!
Not funny!