How to reply to "Thank you" from the man who takes orders from restaurants (like McDonalds), I'm not sure but are phrases "you're welcome" and "don't mention it" OK? They feel strange in this situation.
Phrases like "you're welcome" and "don't mention it" are rarely inappropriate; they show friendly politeness, which is pretty much universally appreciated.
That said, when someone takes my order at a fast-food restaurant, and they say thank you, I often reply by saying, "Thank you."
This is not mere parroting; essentially, what is being said is:
Employee: “Thank you [for your business].”
Customer: “Thank you [for your service].”