I just discovered SO-P-I ... Wow, does it simplify our invoicing!!!
 
The only suggestion that I could make is to allow entry of several $ amounts so that multi-box shipments and/or handling charges could be totaled right in SO-P-I instead of using a calculator, then entering in the total.
 
For example, if the + key is pressed after a $ amount is entered, just add it to the current total, and then stay in the freight field to accept another number.  Pressing enter would move to the next screen field like it does now.