I don't know how it works in the US, but here in Europe we can see the amount we were billed on the card terminal and the shop isn't just able to go change it after the fact.
In the US, the terminal is not brought to the table. The tip amount is written on a paper receipt and entered by the clerk at a (usually) out-of-sight terminal, usually after you've left. A customer doesn't interact with a terminal at all.
This is one of the (many!) problems with how financial technology works in the US. Cards get an auth placed twice for the same bill (one without tip, one with tip), there's little to no card-based security in place when making purchases and customers don't get to hear about their transactions until many days have passed since the purchase.
Every time one of these discussions comes up, it always seems shocking how bad bank/card mediated transactions are in the US. It's as if you just give your open wallet to anyone. I've just moved to Europe and it feels like I've gone back in time a decade. I can't imagine what it must be like in the US.
