I used to import a lot of stuff from the US to Norway. I lived all the way up in northern Norway, so parcels would take roughly 5 working days from Oslo to where I lived.

Domestic overnight mail / express mail was prohibitively expensive, something equivalent to $150 for small items.

However, if I ordered something via USPS International Express, those items would automatically be shipped as overnight / express mail once inside Norway, and handed to the Norwegian postal system. A parcel from New York to where I lived would take 2-3 working days, and as a bonus, USPS Int'l Express only cost around $50 for the same size parcel!

So while not the same type of arbitrage as OP posted about (where items become cheaper due to free shipping), I could save a lot of time and money.

Maybe a more extreme example would be the ultra cheap shipping prices from China. You paid like $1 in shipping, which would have cost $10 if you bought the same service domestically.

IIRC, the root of these practices go back many, many decades. And has a been a thorn on the side of modern shipping ever since Chinese e-commerce exploded.