If as part of your purchase of a used car you also purchased an extended warranty which could be cancelled within the first 30 days for a full refund where you paid sales tax on the extended warranty, you are entitled to a full refund. Meaning, the price you paid for the extended warranty and the sales tax as well under your agreement.When I return things to the hardware store, I get the price I paid and the tax as well. Your situation should be no different.