Almost all stores when then they have BOGO (buy one, get one) free items will ring them up half price at the register. In other words, instead of ringing up the first item at full price and the second one at $0 they ring both items up at 1/2 price. The registers are set to ring up all BOGO free items at 1/2 price no matter how many of the items you purchase.
This great because it allows you to use a separate coupon on each of the 2 items saving alot more money! This great also because most stores will allow you to just purchase one of the items at 1/2 price.
I do this when I only have one coupon for the item or I just want one item.

For example:
Say, ACME has paper towels on sale BOGO free this week and the cost of the paper towels is $3.00. You put 2 packs of the paper towels in your cart and when you get to the register the cashier rings the paper towels up at $1.50 each. You have 2 coupons for $1 off the paper towels. The cashier takes off both coupons and you get the paper towels for $.50 each! Maybe you only had one $1/1 coupon for the paper towels, you could just buy one pack and the cashier would still ring it up at $1.50 and you'd get the paper towels for $.50.

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