DISCLAIMER: I not an accountant or lawyer, nor did I sleep at a Holiday Inn Express last night.

Typically, tax is computer on the sales price. Discounts and rebates are applied after tax is calculated. (Watch how this works next time you use a coupon on a grocery store).

However, this is a tax question for lawyers and accountants, and may even very by state. I'd definitely consult a tax lawyer in your area, no matter what advice you get here.