Our group treats bags of holding and handy haversacks both as having a shoulder strap, long enough to be slung diagonally across the PC's chest. No difference between the two in how they are carried, only in what they weigh and how quickly you can find things in them.
A sack with no carrying straps would have to be dropped every time you needed that second hand for a shield, off-hand weapon, spellcasting* or whatever. That's pretty annoying, and it's hard to see why the crafters of such items wouldn't realize that.
GM allows us to pull the strap over our heads and drop the bag as a free action; this is sometimes necessary because carrying one of the larger types pushes us into Medium encumbrance and cuts our movement in combat.
*Yes, I'm aware there is no rule that says somatic components for spells require more than one hand; but it seems like some of them likely would.