I've been comtemplating buying those myself. I want to finish my 15th century kit before I even think about armour from another time period, though.
I suspect that the two do go together. The photo with the gret helm shows the same aventail underneath it, and the text from the catalog even says, "Normally worn over another helmet," and they can be purchased as a matching set for $441. The lines and general shaping look pretty nice, too, for that price.