I do the same thing. 5 bags seems to be the magic number for me as shipping increases with each bag after this. Sometimes pet360.com has the better price but I think they are just 2 different fronts of the same company.
OP, see if you can find a local farm or feed store. There's one where I used to live that had ridiculously low prices on Mazuri and aspen. It wasn't uncommon to see a large bp or rat breeder hauling off a pallet full in a trailer. It is kind of hit or miss though... now I live near exponentially more feed stores and can't find aspen in a 200 mile radius.