Poshmark's cushy deal with the post office [View all]

As a long-time eBay seller, I thought I was an expert on all things shipping. So I was concerned when my teenaged granddaughter started selling clothes on Poshmark and said she never weighed a package or worried about what box to use. All things get shipped Priority at a cost to the buyer of $7.45. Small, medium, large flat-rate boxes... $7.45.

It finally bothered me so much, I looked it up. Sure enough, Poshmark items ship for $7.45. Below is the (hard to read) explanation I found.

I wonder... Do lots of companies have this, and why doesn't eBay?