Since these are simply envelopes with bulk rate info and Biden address printed on them... a person with surplus envelopes could just scan and print out multiple Biden envelopes to send in. In fact, just printing out a sheet of the Business Reply Mail info and taping/gluing to an envelope would work, too.

I used to work at a business that had BRM and that's how we sometimes received mail. We even received our BRM postcard that someone taped to a large envelope and mailed to us.