I am in St. J. and the only box I know of is downtown and it seems to only be big enough for the flat rectangular boxes and padded envelopes.
I just go directly to the source up by Lyndonville. Really a pain they are only open a couple of hours in the afternoon too.
Where are you at?
Best of luck. This is my first entry in the nationals so I am not too confident. Was not planning on doing so until my wife talked me into it at the last minute. You've got nothing to worry about helping me out. 
I'm in Southern NH but grew up in Windsor, so i know all about small town living.
I sent my first entries to NHC last year, didn't advance but got some great feedback.
Maybe this year I can do better. Good luck.