Well, I didn’t know and now we all know 
When sending a letter abroad, how does the receiving country’s mail service get paid for their work? As in, if I send a letter to the US, I purchase stamps from the post office which pays for its travel in the UK. When the letter gets handed over to the US postal service, how are they paid for their work?
There is a global unified postal service body called the Universal Postal Union . The Union creates a framework and standards so that ALL countries can exchange mail freely without forming individual contracts with every other country.
When one pays for the postage for international mail, a portion of the money goes to the home country’s postal service and a portion goes to the destination’s postal service. The portion of money exchanged depends on how much mail (in weight) each country is exchanging. This is set up so that the destination country receives money for delivering the postage. Countries that receive more mail annually get less money per kilogram of mail.
The fee paid to the destination county is called a terminal due. The UPU is so important it has more members than the UN and a country cannot honestly be called a country worth being taken the least bit seriously without being a member or having a plan to become one. Yes North Korea is a member. In fact, you can send DHL to and from North Korea. They even have an office in Pyongyang you can call. FedEx and UPS do not deliver to North Korea due to sanctions. You can send and receive mail to/from North Korea via USPS, but it is limited to first class mail (i.e. documents and postcards) only due to sanctions. For parcels, DHL is the only option. DHL is allowed to operate in NK because it is presently a German company and not American.