sum invoice product field in invoice and create formula in invoice product

Hello; 

I want to create a field in invoice product that when I choose the specific product for existing product then make a calculate discount in the record and after that sum the discount of items in invoice record. Please tell how can I do this?

For example I have 2 product groups blanket and matters when I select blanket group items make 10% discount and when choose matters make 15% discount, after that sum discount amount in invoice record that I can print it.

Thanks