Hi Kim
I can't help really with the postal issues other than say if a certificate is returned then the person could maybe contacted by email, saying it has been returned. I know you already have those details for sending out pack information. Advise them that if they wish for another one there will be a fee for re-printing and sending by recorded/special delivery or sent by pdf.
I know this is more work for you and a bit of a faff.
With regards to PDF files being altered by unscrupulous people, yes they can be edited, even if locked. Someone will always find a way, especially if they have the correct software and determined enough to do it.
Even a hard copy can be changed, a good scanner saved to a pdf file and edited. So easily taken from a website unless that has been locked in some way. I know your website is because I tried to copy your postal address to put in AA route finder. I'll have to put that on my website when updated!
I sound like a crook, seriously, I'm not, hubby is a printer and edits artwork all the time. Even with the Apple and no software it can be done, although a lot of hard work for anyone.
Maybe as suggested incorporate postal charges into your fees but not until I have done my course with you, :Grope: