Point out an incorrect charge on your receipt and request a refund for the price difference.
Excuse me, I just looked at my receipt and I think there is a small mistake.
Oh, I'm sorry about that. What seems to be the problem with it?
I was charged for an extra vanilla pump, but I actually asked to have it removed from my drink.
Let me see. Ah, you're right. It looks like the system added the charge instead of taking it off. I apologize.
That's okay. Is it possible to get a refund for the difference?
Of course. It's only fifty cents, but I can put that back on your card right now. Do you have the card with you?
Yes, I have it right here. Do I need to insert it into the machine again?
Yes, please. Just tap or insert it now. There we go, all set. Sorry for the trouble!
No problem at all. Thank you for fixing that so quickly.