Doors are at 6 for the Boston show and the show is at 7.
http://www.houseofblues.com/tickets/...?eventid=79820
They turn HoB into a dance club at 10:30.
http://www.houseofblues.com/tickets/...?eventid=80660
So I would say DIIV at 7 and Htda at 8 or so...it should be done by 10 is my guess having been to Saturday shows there before.
I think merch is handled by the venue or at least by local contractors, so it will vary from location to location. For instance, the same guy who sold me a shirt at the Wave Goodbye NIN show at the Aragon also sold me a shirt at the Final Fantasy: Distant Worlds concert at the Rosemont Theater (both in Chicago). Cash is by far the safest and fastest method, so I generally hit up an ATM before going in anticipation.