You give them your email address that you used to sign up with PayPal, and they login to PayPal, click the Send Money tab, then enter in your email & how much they want to send you. Make sure they send it to you as a gift, and with a note attached usually along the lines of "This payment is for items I have already recieved virtually and I verify that I am the owner of this PayPal account with this following email, and that I will not charge back."
It's possible for them to refund their money, it'll decrease your chances of being refunded alot though, if you do the above.