The way to use OpenERP for this need is to add a line in your sale order that summarize the postage cost for all products. You can install the "delivery" module so that OpenERP will even be able to compute the cost for you automatically based on delivery grids.

Example for a sale order with two products:

Product X      3 items     $50
Product Y      2 items     $20
Postage cost                $5

Total                      $75

If you want to do the way you proposed, you will have to develop a specific module. If you do that, I would not suggest to change the logic of the current fields, but rather add new fields that fills in the current fields using on_change methods.