
You can actually get both bottles of shampoo (example product) for free when the store has a buy-one, get-one free sale. A combination BOGO coupon plus the BOGO sale is all you need. When the first bottle rings up it makes the second one free AND the coupon makes the first bottle free.
If the store insists you purchase an item, buy three bottles. In this case you will pay for one and get two free. Still a DEAL!
Happy couponing...