I just got back my first 2 rolls of 35mm E-6 from Walmart. On sending them in, I checked the special instructions box on both envelopes and wrote in "E-6 processing. No Prints. No Scans. No CD." The first roll came back mounted in cardboard slides. They look great and no scratches (I have only viewed them holding them up to a light. There may be subtle scratches I can't see)
The second roll was incorrectly processed. My guess is they put it through C-41 without reading the envelope. They are clear with what looks like B&W image on them. The slides are cut 4 slides to a strip. They also included a best attempt black and white print, a "The machine did it" letter, and a disposable camera with Superia 800 in it. I send RVP100 and they compensate with Superia 800. I will be calling to see if they will do any better.
If you go the Walmart route, write E-6 all over the envelope.