Ask HN: Web API for Payroll Withholdings

I'm working improving the payroll package in my company's not-for-profit accounting software, and I thought it might be nice to add automatic calculation for federal and state withholdings. I figured that someone might have an API available which would return the appropriate tax tables in some sort of generalized fashion, but I've been unable to find the only such service, whether freemium or commercial has been the one at http://www.suburbancomputer.com/tips_state_tax.php which does not appear to offer any sort of API, but simply consolidated and compressed tax tables.

Does HN know of any such service?