Steve:

Just to make sure I'm on the right page:

Gross annuity amount is $1,000 but then withhold Federal Tax $150 and State Tax $100 so the actual check is $750.

As the gross never changes then I assume the withholding never changes (until the tax rate changes).

Why can't you set up the invoice import as a Header/Detail, each header then has 3 details (Gross annuity positive, Federal and State Taxes as negative)?

If this was Canada then I would use Payroll to accomplish this as tax rate changes would be automatically handled.