How Freakin' Coincidental that this thread would be resurrected today!! Just had a call this morning on a MP3555 where they were having issues printing envelopes (text in wrong orientation/ jamming).
With Auto Detect set on the bypass it would always detect 5 1/2 x 8 1/2, and even with bypass set to "Driver/Command", it would still keep the 5 1/2 x 8 1/2 setting. It would pass the envelope without jamming, but the text would be shifted to the right and slightly up on the envelope.
I tried the PS driver, and it worked fine!! (Still don't understand that). I didn't keep this as my "fix" though. What I ended up doing was setting the bypass for envelope, middle thick, and 4 1/8 by 9 1/4 (or whatever it is for COM10 size), and left the bypass on "Driver/ Command". They Only print envelopes through bypass, so hard setting that size works in this case, but doesn't work in other cases.
Just thought I would share.