When you add products, you can set up a purchasing Box Quantity (number of items per box) and Carton Quantity (number of items per carton). Go to File > Product > Master: Setup, Purchasing.
Then when you are entering orders, you can specify that you want to order the Box or the Carton Quantity by typing the number followed by b or c in the Quant Ordered box. This is particularly useful if you know you want to order in bulk, but you don't know the precise quantity in a box or carton.
In the example below, typing 10c into the Quant Ordered box the results in 10 cartons of bleach being ordered, which translates to 120 units (12 per carton). Inform automatically calculates the Price Extension as well.