Hm...from what I recall on other members' posts, there's a space in Paypal(whilst donating or I think after, I don't know) where you leave for "comments". In that comment area, I believe you list your username, and then he'll know that way.
Though I wouldn't know as much as Rukario would, to be honest. x_x; The donation process is really weird to me, so yeah. xD