In the olden days, you were able to overnight a 200 pound item for 4 bucks. They lost a lot of money. Been this way for while.
FWIU amazon doesn't pay per package. They negotiate a set amount for as many packages as they want to ship.
For example, Amazon gives UPS $5mm in the beginning of the year, which in turn buys them the rights to ship as many packages as they want. (didn't really word that right, but you get the point)
CMIIW.