Comment by yujzgzc
It's not arbitrage until you can make money by selling something that costs you less than what you bought it for. What it is is bundled product (item + shipping) being priced lower than just one of the elements in the bundle (shipping) therefore making a case that one might as well always buy the bundle.
Yeah, "arbitrage" is not the right term here. This is just a complicated way to get a lower quality version of a service (sending a letter by mail) at a lower price.