When I worked at the post office (a long time ago), I operated one of these mail sorting machines. They would jam once about every 5 or 10 minutes, and in the process mangle the letter(s) that got caught in the jam. Usualy they were cassett tapes or anything that was more than thickness of 1/2 of a centimeter or so. It is strongly advised to not send anything but a letter in an envolope. USPS treats envelopes differently than other packages.