loop auto-increment issue on every fix value

I try to make inventory and sales based project but in this if a product has freight (transport charges on per carton), if carton=18 pieces and if some one buy 2 or more cartons LOOP give me auto freight of cartons on every carton but if one carton and some pieces are entered it only give me only one carton 's freight(freight due on complete carton only not on piece). I tried IF with FOR LOOP but i can't successful.