I always used Walmart for the 1 roll per year or so of 8mm film that I shoot. I just drop the can in the mailer, write "8mm color reversal film, process E-6" on the mailer. I place a white label on the can that says "Exposed 8mm color reversal film, process E-6". The can is, of course, taped up.
This has always worked, but the last roll I sent out was sent back in a bubble mailer with a sticker saying "we do not offer this transfer service". The bottom of the can was cut off (yes cut, I guess they couldn't bother to remove the blue tape) and missing, and my film was sitting in what was left of the can, wrapped on a different spool than the one I sent it in on. It was in a normal, non-lightproof bubble mailer. I assume it is ruined.
I am going to call them later and see if I can at least get a replacement roll of film.