If I were you I would try to do this:
- Get a account system for players built into the game
- Set up some sort of site where you have the player login to their ingame account then after they login you can now allow them to buy offers and perks and stuff. (To check if a player has a certain perk that they purchased on their account make a value in your account database that accounts for that perk or item and then tell your game to reach that certain database server of yours to see what the player purchased each time they log in, make sure you clear their perks and items if they are offline to prevent cheapskates).
I know that you want to get premium stuff into your game but setting the microtransaction up is just the beginning, your going to have to deal with people trying to get things for free and taxes. Also I forgot to mention, most indie games run at a loss and some don't even generate money.