I FOUND THE RIGHT ANSWER!
And it's a totally stupid one.
My 1099K had cents amount on it. TurboTax rounded it up -- and got tripped by its own rounding. If, when entering 1099-K, I enter the rounded amount in the first place, everything works!
This looks like a TurboTax bug, with a fairly easy (but unobvious) workaround.
