ELI5 Postage Stamps - does the sending or receiving country get money from my stamp?

ELI5 Explain Like I'm 5: Bite sized answers to stuff you should know about - in a mini podcast

If I send a letter or postcard from the UK to the USA, who gets money from my stamp? Once it reaches the receiving country, what makes them obliged to deliver my letter if they are not receiving payment for it? When was the global system for postage established? How does overnight shipping get where it's going faster than a normal package? Why not make everything overnight?

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Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: riconquer2, riphitter, rhynod, psunavy03, prudent-artist-274, p33k4y, arcturusflyer and admirable_cry_3795

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