A certified receipt doesnt prove anything. It only proves you sent something that required signature, and because someone might have to sign for it on the other end, they might actually do something with it. Besides, I dont know any PD in MA thst will renew over the mail, dont they all ask for an interview? - maybe I always got lucky.
That is assuming you even get the little green paper back. I think I only get about 40% of those back.
The Commonwealth of Massachusetts has clear direction on their website which states applications can be mailed in. This is supported by law.
Apply for a firearms license
A firearms license is required to possess or carry firearms in Massachusetts.
www.mass.gov
Time for police departments to start following the law.