Welcome to FlyerTalk,
antjef.
Originally Posted by antjef
I ... followed their instructions to drop the form in the mailbox on site. ... I don't have any proof that I received the stamp and that the form was mailed.
'Instructions'? They told you to do it, or they advised you to do it, or they suggested that it would be convenient for you to do it, or what? I myself would certainly never send any kind of form like that without making a photocopy first (for this very reason that you have learned the hard way).
cheers,
Henry