[yencabulator]

I think you just didn't look deep enough. It says "Look up your destination country in the Index of Countries and Localities to make sure you’re following their rules." and the rules for Sweden say

> [...] paper money [...] are prohibited in Priority Mail Express International shipments to Sweden.

So don't pay for Priority Mail Express, just First-Class:

> First-Class Mail International is a generic term for mailpieces that are postcard-size, letter-size, or flat-size and weigh less than 16 ounces [...]