I need some help adding quantity per case. Here is a quick break down of what I am trying to accomplish.

Product A comes packed in a case of 48 pieces and weighs 17 lbs. They must purchase a minimum of 144 (3 cases) but can order any quantity over that, and it does not have to be in units of 48. For example, they could order 145, which means they would have 3 boxes at 17 lbs each and 1 box at 1 lb. I need to be able to list the weight so that the UPS shipping estimator gives them the correct breakdown in price. Currently the UPS estimator has a max weight value per box of 50 lbs, which means anything under 50 lbs gets listed as 1 box, but this is incorrect. Is there a field in the product or inventory section that I can note how many pieces are in a box so that the shipping estimates will come up accurate?

Any help would be appreciated.