At first I thought this was brilliant but on reflection, I am not so sure. One of the reasons I’ve heard that postal mail trumps email is the postmark. It is a reasonable heuristic with which staffers can filter the correspondence from real constituents vs. anything from anywhere else. I cannot imagine MailCongress being able to preserve the postmark, not at one dollar a letter.
Beyond that, I think hacks and workarounds like this are delaying better considered solutions. We need to develop a good enough means of verifying constituency that lets citizens choose their own preferred means of communication. We have so many great ways to communicate with each other, why should we be so crippled when exercising one of the most basic elements of participating in an open democracy?