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.
Product Jims Book
Carton Field is 10
QTY available 150
Carton available output display 15
Invoice / Order
Customer orders 100 units of JIms Book
Cartons / QTY