The rules allow you to send an email address to an existing buyer so they can send a payment via PAYPAL.
What I find a LOT easier (and less risky) is I just create a "special lot for PARTOFUSERNAME" ebay lot that they just purchase and pay for in the normal way. I do this all the time for special requests and additional payments (my store is closed right now for summer holidays). You'll have to add a picture to the lot, can be a picture of anything, I sell in the stamp world and I just put in a picture of the back of a stamp.