From this website, I found the following advice:
If you have an introductory word (or two) that is being used as an adverb (usually answers the question of when, where, why, or to what degree), then no comma.
They give this example: "Tomorrow I’m going to get my hair done."
I thought this was the reason "and so" is often written without a comma. This is from the fraze.it search engine:
And so your ending will be sort of a reflection back on how you saved the world.
But then I remembered that "and so" isn't an adverb but a conjuction ...
Why do people often omit the comma after "and so"? Is there any grammatical rule?