I bought the small one for my FasTrack (SF Bay area version of EZ Pass) but it was too small, so I got the large one, which is a bit big, but it works for me. Both of them velcro pretty nicely to a standard messenger bag strap. (Posted on 8/18/13)
I have always had difficulty when traveling toll roads and bridges to access my EZ Pass. In the past I have taped it to the windshield which doesn't work, tried to dig it out of my pocket with one hand which is extremely dangerous.
I own one of your courier bags and recently purchased the small zippered pocket which easily attaches to the courier bag shoulder strap and voila- a no hands system for the EZ-PASS which is easily removed and doesn't have to be left on the bike. The small is too tight and recommend the next size up to enable it to be carried in a horizontal fashion like ones mounted on the interior of the car windshield.