What I did was to report it on the "Enter Business Expenses Not Yet Reported" page. That comes after the "Enter Your Business Expenses" page which has no matching category. I described the expense as "Sales tax collected and reported on 1099-K".
TurboTax should have some way of doing this on the 1099-K page -- i.e. ask you if the "Gross amount of payment card/third party network transactions" includes sales tax, processing fees, or any other non-income item typically reported in box 1a.
