Couponing like a mofo, nine-cent tax refunds, and other things that make you go hmm

Free shampoo, free salad dressing, free barbeque sauce, plus big discounts on a bunch of other stuff.

Anyone know how I managed to get negative tax? Does that happen when the amount of the coupons on taxable items exceeds the amount of the taxable items? This was the case this time, and I can see why the register might make that happen, though it doesn't actually seem correct based on how sales tax works between the store and the state (I used to handle such things for a store in a previous life). Kind of amusing though.