Is it possible to actually link clients to individual invoices, on PayPal? I'm hoping to use it for an invoice backend system on my website.
I've looked everywhere and literally can't find a solution. I have seen it working inside of WHMCS before.
You mean an invoice created on PayPal's site? Sure its possible. Just send a link like this (with the actual invoice number in place, obviously):
https://www.paypal.com/us/cgi-bin/?cmd=_pay-inv&id=INV-NUM-123-456-789-012