When packages are dropped off at a FedEx or FedEx Office location, are FedEx Envelopes handled separately from Pak's and are Box's handled separately from everyone else, in order to prevent damage or do all three types end up in large bins eventually together like they do at the USPS (Envelopes, Padded Mailers and Boxes end in large bins together)

Thanks in advance for any insight!

I've been wondering for a while now why shipping with Fedex Pak mailer bags is cheaper than using Own Packaging.
Does anyone have any idea why?
As long as you are under the 5.5 lbs weight and 940 cubic inches volumetric limit using a Pak mailer bag is often a good 20-30% cheaper than own packaging, sometimes I just put my item that's already packed in "Own Packing" inside the Pak bag to save 30%.
There must be reasons for this to be the case?

Just to add my experience regarding this is with International Priority Shipping if that makes any difference.