Upload & Sell: On
Simply give him a letter, in standard 'classical letter' format, with your and his name, address and etc. The letter could say something like "Invoice for 'name of lens', for a cost of $'dollars', payment in full required at time of sale', 'thanks and etc' , 'yours truly'.
Then, when you get the money, write on the letter (in white space at the bottom), "Paid in full, 'date', 'your signature'. You're done!