I need to enter Sales Orders in Packages but Invoice in units. Is there any solution in Odoo settings?

Our business requieres:

• Inventories in Packages (x6, x12, etc) - each product has only one option of package, x6 or x12 but never more than 1 option
• Remito (legal document for transportation) in packages 
• Sales order in Packages 
• commercial invoices in units

So far I was able to get an aceptable solution with the following settings:

• Product => general info  => product price by package (to make it the main unit of measure)

• Product => Sales => Packaging : Unit - to be able to invoice in Units
• The sales order start indicating the product in packages
• and then it needs to be manually changed it to units (in order to invoice in units)

The main problem with this solution is that entering the sales orders take a lot of time and changes with potential errors.

thanks in advance to all !