[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.

[kmoser]

Theoretically you can offer a service that sends a physical message for less than the cost of a letter, and use this hack to do it profitably.

[stevage]

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.