How do I make an in game shop?

So right now I have the game saving the coins to a global variable, it works perfectly. Now on my shop screen I have two characters, the main one that you get for free and then one next to him that costs 5 coins at the moment to purchase. When I push the button to purchase him the game takes you back to the main screen where you can start it up with the newly purchased character. But when I go back to the store and select the free character and then go back in the paid for character now costs 5 coins again. How do I make it so this doesn't happen and when I purchase him he is always purchased? Thank you in advance.

You can set the purchase button to a variable. Once pushed, change the variable from, let's say a 0 to a 1. Make the purchase button only clickable when the variable is equal to 0.

Let me know if this helps.

is there any tutorial as how to use IAP behavior ???