Add Carton field to Product Inventory, so how many units go into a carton. 

Functionality to then display using the QTY how many cartons are available instock.

Functionality to display carton QTY on invoice.

IE. 

Product Jims Book

Carton Field is 10

QTY available 150

Carton available output display 15

Invoice / Order

Customer orders 100 units of JIms Book

Invoice Displays

Cartons /  QTY
10            100