I reckon you just need to ask them what the part was and what the shipping address/email address was and who placed/took the order....
I find it is pretty unusual that a business would send a part without the payment being received and cleared?
It smells like a scam to me? Maybe someone has hacked his customer list and told them all they owe $75 hoping some will simply pay it?
I think you need to let him know he might have been hacked? Forward him the email, or scan and send him a copy of the letter.
He probably knows nothing of it....
Oh, I forgot.... you owe me $75 too...