Hi cbr,
When me and my wife received our RFE’s separately, there is one small pre-addressed USCIS Envelop on each of our RFE envelops, however that is not sufficient for me to squeeze all the necessary documents to send it to USCIS…. I took a big (A4 page size) USPS envelope and send all of my documents including empty USCIS pre-addressed envelop in a priority certified mail. (Delivered in 3 days)
I believe when they say “special envelope” means, you could use separate envelop which can accommodate all of your documents freely (as mentioned above)….This is only my opinion and this what I would do...hope this will help.
Good luck !