329
generulesity
(lemm.ee)
"No one who works here at CapitalOne would ever tip this much so we just wanted to double-check you were of sound mind when you did this! :)"
Not sure why you weren't billed for the tip in your story. Having to write the tip amount down on the tip line of the bill is 100% how it always works in the US. You may have written it on the customer copy of the receipt, perhaps.
It's because unlike with american cards you have to confirm the transaction on the card reader while it shows you the amount (with either a pin or signature in some cases). After you confirmed it the transaction cannot be changed, i.e. the tip cannot be added. So the american way of tipping does not work with foreign cards.