Switchdiscs doesn't provide any mailers but they do recommend certain ones. I guess she is just referring to the mailers members use? The mailers of choice at switchdiscs are bubble mailers when sending the case and the thick paperboard mailers when sending just the disc or the disc+artwork with the disc in a paper sleeve too for added protection.
On the help page they say to use these ones:
Paperboard Mailers
I use a 6" x 9" white envelope that I can print the addresses on and then put the disc in a plastic protective clam shell case and slip that along with the artwork in the one envelope. That has worked pretty well for me. I also use the paperboard mailers and have labels for the addresses. Both work great because they fit the artwork without having to fold it across the artwork itself.