Most of us were taught to place a comma before a sentence-ending “too”:
We’re going shopping, out to dinner, and then to a movie, too.
But is that comma really necessary? “Too” in this context means “also,” but you’re not likely to see the sentence written like this:
We’re going shopping, out to dinner, and then to a movie, also.
No one seems to know how this particular quirk started, but it’s firmly entrenched in our over-cluttered writers’ brains. Even journalists do it, and modern-day practice is to strip news stories of as many commas as possible without hopelessly obfuscating meaning. Still, that niggling comma before “too” persists.
The editors at the Chicago Manual of Style share their opinion:
Use commas with too only when you want to emphasize an abrupt change of thought:
He didn’t know at first what hit him, but then, too, he hadn’t ever walked in a field strewn with garden rakes. In most other cases, commas with this short adverb are unnecessary.
The bottom line is, there’s no clear rule that either specifies using the comma or forbids it. It’s the writer’s choice. The rules of grammar don’t often allow writers to have choices. It’s kind of nice to be thrown a bone from time to time.