What is the correct grammar/punctuation for saying, "Thank you"?
You can either use a period or an exclamation point.
When it comes to punctuating "Thank you" you can either use a period "." or an exclamation point "!", there's really no right or wrong choice here (as long as you don't use a question mark). It's mainly a matter of personal preference based what you, as the author, is trying to convey. If you're attempting to convey excitement, I personally would opt for an exclamation point (but you could use a period, if you'd like to). If you're trying to be cordial (as in business writing), your character is attempting to be sarcastic, or they're deadpanning their speech then I'd opt for a period.
I hope this helps!