editable final invoice

Version 2.0.3.1
We are looking for a editable cart
when people place their order we print out a shipping list, the order gets pulled and then we would like to be able to adjust the amounts for example a 2 Lb product is ordered but after we pulled it it turns out to be 2 .3 so we need to be able to adjust the final invoice to change it to 2.3
Any suggestions?
Thank you!