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It's rather disappointing that I had to pay for this game, and then need to pay again(possibly repeatedly) for coins if I want to enjoy most of the content in this game.

You don't mention which version of the game you are playing, but the version of the game that you pay for is for Windows or Mac (desktops or laptops). There is no facility to buy coins in those versions.

The versions where you can buy coins are for the iOS and Android systems (phones, tablets, etc.). They are both free.
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To err is human; to arr is pirate.