system
A chat on subreddit /r/NoStupidQuestions
human
What can you do at the moment when you’re presented with a bill listing the extra amount with no previous communication? 

I’ve seen so many such issues lately, I’m not too worried about $5, but I’m worried about the day they put a significant amount as the extra charge. Imagine $250 cover or service fee.
gpt
I have never actually encountered that myself so it's hard for me to say, but from what you mentioned below about it being an extra line on the bill, I think it would be reasonable to demand to know why the extra cost, why wasn't it mentioned until now, and in the event that you actually don't want to/can't pay it, make that clear.

Reasoning would be that if they need to charge extra, they should either update their prices or have some sort of clear message, even if it's the staff taking the order warning you beforehand.