Tagging invoices with salespeople commissions

Hi, we pay salespeople on a set commission of a sale. Problem is, they get a different percentage per total sale depending on product sold. Eg SEO would be paid at 15% and Web Design at 10%.

I was playing around with tagging a name on an invoice (so I can extract all tagged invoices monthly to pay commissions) but wondering if someone can think a bit out the box on how I can do this within the fields available.
Basically, A code per sales person, then a separate code or a percentage number per service. With these two tracked we can export to excel and get the data out…
Finally, when we copy invoice or create recurring invoices, those same codes must follow through.
Any ideas on how best to do this?