This is one of the big holes in the zenfolio/mpix fulfillment; when they refer to matting, it is backing board mat only. there is currently no option to have the kind of mat that goes in front with the beveled hole cut to show the print. This is true for framed prints as well. The ordering process for this is very confusing to store owners and customers alike.
MPIX has announced version 2.0 of their service that that that does allow the kind of mat you are looking for when you order a frame, but I don't know if you can order it like that without the frame. Zenfolio has stated that they will be implementing the new mpix 2.0 ordering options to Zenfolio 'soon', but who knows how long 'soon' is for them... could be next week, could be 6 months from now. I imaging that all the changes are not an easy addition to add and debug for us.
Perhaps someone from Zenfolio can answer these questions before too long.