In a case like this, you should bring cash with you anyways. It is a different situation than going to a restaurant. When you travel one should always have cash, even if one normally doesn't. It's common sense.

Personally, I just stopped doing that.the tips were small to begin with.

I think people also know non Americans aren't going to get it right. If they're the type of person to get offended over that, I wouldn't spend my time worrying about their opinion of me