Add a feature for setting "Buy One Get One Free" offers as well as offers such as "Buy 10 get 10% off". I would suggest setting up the offer on a dedicated "Offers" page in Back Office and then setting items against it (which would allow multiple different products to be set against a particular offer allowing for offers like "Buy 2 pairs of socks and get the cheapest 1/2 price"). It would be useful to be able to apply an entire category to an offer (or the ability to easily select large qty's of items like in a wizard).
If setup as an "Offers" page in Back Office then when viewing an item record it would be good to view all the offers currently affecting that product.
When scanning items in to the till that activate an offer it should pop up with a message to alert the cashier.