Hi,
Had a thought that if the briefcases were lighter, for what ever reason, more letters and keeping of letters might be encouraged. Either that or make it possible to hold more than 6 sheets of parchment, though that seems problematic from keeping it from being a bag of holding... Unless... you could make it so anything placed in this "bag of holding"ish briefcase increased in weight dramaticaly, so that effectively only paper could be put in there... Just some stupid ideas.
Cheers
Threnn