The standard form of Soviet war correspondence during WWII were letters folded into a triangular shape.
During the war, the mails were brought for free from the front to home. It could not have been differently, because probably the postage stamps would have been the last item the halting logistic support would have delivered to the front. Even so, postcards and envelopes were shortages. The soldiers’ genius has thus created, right in the first months of the war, the format that was a letter and its own envelope in one. The folding process is very similar to how we, in our childhood, folded our soldier’s shako, knowing nothing about the triangular soldier’s letters.