I have no idea why sellers would do this with eBay fees and USPS small package shipping costing well over 13 cents.
Now you're part of their education.
Or... they are sophisticated and trying to get a ton of relatively inexpensive positive ratings before selling things that are actually expensive?
I ended up with an enormous overflowing mountain of packages every day for weeks. I might have gone crazier, but there was a serious bug in eBay's checkout. Try checking out with 400 items in your cart. It really gets upset.
99% of the packages were Chinese sellers but the packages all came from Mongolia, so there must be some sort of postal arbitrage going on there.
It was all random stuff. Hairclips, 500 bicycle lamps. Dozens of tubes of ICs of every flavor. Crazy times.