It's not cheating. Some shops do charge taxes according to the pre-sale price. Costco is one of those. But it states it clearly on their flyers and coupons.
That's why for hair care, I like to go to Japanese operated places. They have never failed me and always write the the price and their break-downs on paper to show you for approval before they start. Not to mention they place a towel on your face when they wash your hair and the entire store saying good bye and thank you! Best of all, these little extras don't cost extra!