I'm not saying this is a genuine article, but I think the seller is misspeaking (miswriting?) when he calls it an envelope. He also calls it an envelope panel. It looks to have been cut out of a larger piece of paper, rather than an envelope as we know it.

] I highly doubt so since it's known that envelopes didn't exist in JA's times, letters were folded in such way that the leaves themselves were the envelopes. The use of envelopes began in Victorian times, when paper became less expensive.