Update sales order after picking but before "fulfillment"

Hi everyone.  Let me explain my situation.

We have a custom column field on our sales orders called "Carton Count".  This field is an Integer field that is designed to hold the number of full cartons -- that is, boxes -- that are being sent to a customer.  For example, if an items is stored in cartons of 10 pieces, then a sales order for 30 pieces would have 3 cartons.

The primary purpose of this field is to help out or packers.  They will use this carton count to reduce the amount of time it takes to check and pack the orders before they are shipped (it's easier to count 3 unopened cartons than it is to count 3 cartons, get the quantity that's in each carton, and multiply it out).  The problem is that when the order is picked, we can't guarantee that the quantity picked is going to be the same as the quantity committed.  As such, this carton count field needs to be updated