I sent a half-dozen PFDs CMRRR between 3 baddies and after getting no response at all, I decided to send future ones with a stamp. I'd much rather save the $6 or so per letter.
ETA....I view PFDs as GWs for unpaid debts. I don't want them building up a mental wall upon receiving my letter because they had to sign a green card.
Those are two excellent points I had not even considered. Thank you for the info.