Why not prepay groceries?
newy: For the budget conscious - having to commit a set figure each month might not be an appealing option (they're already locked into rent, electric bills, etc).  For the not-so-budget conscious, the discount might not be worth the effort to set up this system and lock yourself to one grocery store chain (people like variety - one week Safeway, the next Trader Joe's).  The grocery-space does need improvement though - who knows maybe it could work?