Sell items by pallet

I need to setup a cart that shows the price per sq. foot as the price in the list pages, then when you click through to the product description and actually add it to cart, it only lets you add by PALLET. So for instance, it will say "$3.69 per square foot!" but adding "1" to cart adds a whole pallet.

In a perfect world, this would be accomplished with an extra box where you add the product information where you can say "add by pallet?" yes/no and then state how many is in a pallet (120) and then it does the math.

Anyone do a mod like that before? It doesn't have to be PALLET per se, but that idea! I've tried searching the forum but I'm running out of synonyms for "pallet."