To add to #1, I personally had a mental blocker, because I didn't always have enough sats on me ready to go, to cover the full bill. Obviously, try to avoid this scenario, but if you do find yourself in this position, you can ask...

...while paying using traditional money. I find merchants less annoyed, if I ask this question, while paying via credit card. Because we're both standing there anyway waiting for the terminal to update records in the existing "system".