The game is really simple, its a resource collection to fill a requeriment bar on each country and the upgrades speed-up that action.

It tries to tell you a story about a lost letter, but when you find out the country the story abruptly ends, i would guess there's something for collecting all the stamps from each country, but as soon as you fill the bar on the country the person you was trying to find is the ending dont let you continue your game, so for that one would need to slowdown and gather each country before filling that country to 100%.