We sell products in 'flat-pack", very similar to what you would find at Ikea stores.  We want to be able to have an option to charge an assembly fee for any one of our products.

For instance, to purchase a widget flat-pack is $100, and an assembled widget flat-pack is $125

I know that I can add the fees as line items, but different flat-packs have different assembly fees, so we would like to be able to do this per line item.  We would also want to do this on the Website so customers can purchase assembled or non-assembled.

Environment

odoo 9.0c-20160111 (Community Edition)