Hyphens – When to Hyphenate In-Place

Ah, I presume you mean you looked up thank-you, which is an existing noun but not the same thing at all as the common idiom thank you.

Merriam-Webster tells us:

Full Definition of THANK-YOU

: a polite expression of one's gratitude

Origin of THANK-YOU

from the phrase thank you used in expressing gratitude
First Known Use: 1792

Now, the phrase “thank you” is indeed always written without a hyphen.

But when we transform phrases into nouns, we use (often) hyphens to indicate that those words belong together as a set phrase, and together they become a noun:

After the wedding they sent thank-yous to the guests.

As with many nouns, we can use thank-you attributively ("as an adjective") to modify another noun:

I sent her a thank-you note.

A comma after a coordinating conjunction (and, or, but) is only acceptable when the comma is the first of a pair of commas bracketing a ‘supplement’—a phrase or clause which lies outside the main structure of the sentence and interrupts the flow of thought.

  And, as if that were not enough, he went on to sneer at the victim.
     ^---------------------------^  

  But, as you said, there is no point in trying too hard.
     ^------------^

In other circumstances the comma is effectively ‘disjunctive’ and subverts the joining effect of the conjunction.

This counterproductive comma is becoming very common in contemporary business writing; I suspect it represents a hypercorrection by people who have misunderstood the need for commas bracketing supplements, or think it lends formal ‘elegance’ to their writing.