i need to add a 20% service fee

[schiuma]

I have a problem i need to add a 20% service fee to the TOTAL not the products of the sale, and I need to add a 2 fields, since i work with telemarketers selling my products, i wish to include their employee # in the order email notification, so i need to add a fill box for that purpose or a check box if no salesperson helped the client...if anyone can help me please :)