Look, if there’s no sales tax listed on the invoice, it means the tax liability has been shifted onto you. That means you’re on the hook for the tax itself, but you also get the benefit of claiming it as an input credit.
You report that sales tax in your quarterly filings under sections II. 10. and III. 10., plus in the standard sales tax return form
Here’s the guide for the sales tax return
You can find the rest of the instructions right here:
http://www.irs.gov/sales-tax-guidelinesI’m not entirely sure how this works for individual freelancers, but if you’re running an LLC, you’ve gotta book all of this in your formal financial accounting.
Invoice: 4.... / 221...
Sales tax: 1... input credit / 2... sales tax payable (honestly, I don't know which chart of accounts you're using)
You clear out the vendor account once you pay them. As for the input credit in account 1 and the tax payable in account 2, you clear those out using a journal entry at the same time you file your monthly tax return. Basically, by clearing those class 1 and 2 accounts, you’re either showing what you owe the government or showing a credit if you overpaid.
Then, when you actually pay that amount or when the IRS sends you a refund, you clear that obligation or credit using your bank statement as the basis—either when they cut you a check or when they apply it to your account.
If you just show a credit on your return but don't actually ask for a refund or a direct transfer—maybe you just want to leave it as a prepayment—then you just let it sit in the appropriate class 1 account until you actually use it.