How to add in game store?

I'm working on a 2d endless runner for android, and it's pretty much done, i just want to add an in game shop to it.

Basically, in my game the character gathers coins and i'd like the user to be able to use those coins to buy skins for the character, but i'd also like to have monetary transactions. (like buy 100 coins for 0.99$) I haven't really found any good tutorial to follow, so i'm asking here, i'd appreciate it if anyone could help me with a tutorial, or some documentation on this.