the Universal Postal Union (UPU) arrangements have been around since 1874 (originally General Postal Union).
Parties to the agreement will process and cover costs of delivering mail that lands from overseas countries, with postage costs set and kept by the origin country.
The settlement of imbalance of mail volumes is reconciled through Terminal Dues.
The Wikipedia article explains it quite well, including comments about Terminal Dues and the effect of COVID and explosion of online commerce.
https://en.m.wikipedia.org/wiki/Universal_Postal_Union
"In 1969, the UPU introduced a system of terminal dues. When two countries had imbalanced mail flows, the country that sent more mail would have to pay a fee to the country that received more mail"
(good to see you here fixnwear)
