How is postage for an international letter divided up amongst the different postal services?

I have always been wondering how postage for international mail is divided up? Who gets how much of the revenue from a stamp? Let's say I am sending a postcard from Australia to Spain. So the Australian postal service is responsible for getting the card to Spain and then Spanish postal service delivers it to the recipient. Who gets what percentage and who determines it? I know there is an organization called the "Universal Postal Union" that is charged with setting international postage rates and defining standards for postage stamps, but I cannot find any information on sharing of postage revenue between different postal services. Thanks for your help.How is postage for an international letter divided up amongst the different postal services?
I've worked for the US Postal Service for over 30 years and have had the same question. The UPU is a mystery that is never talked about.



Have you looked at upu.int? It's a very comprehensive site but shows no clear answers on your question.



From all the information I have had read is that Australia gets all the revenue and Spain gets none if there is a rough equivalent in tonnage sent between the countries. It gets really complicated when it comes to parcels sent. That is why international postage is pretty much standard for letters and differs widely for parcels.