I'm surprised if that's the cheapest method possible, but who knows. Pretty ridiculous when you consider the actual cost of international shipping amounts to local delivery at both ends plus flight. On a good day you can still ship a 300lb human from NY to London with 50lb of books in a suitcase, and another 20lb in their carry-on for $200-$300.
The majority of international mail is actually carried in the cargo holds of commercial airliners. I wouldn't be surprised to learn that USPS contracts with carriers are effectively subsidizing low ticket prices for passengers.