So I've read about itch.io getting a share from my income (donation, game sold), how about in-game purchases, if it's sent directly to my paypal account how does itch.io take a percent of that?
I’m sorry, I wasn’t sure you were talking about your own game(s?). For that answer you’d probably have to ask an admin.
While I’m sure they would accept… voluntary transactions(?) from you and just trust your math, I’m kind of doubtful they would have some sort of universal plug-in that fits all game engines and keeps track of earnings.
All that being said, personally I view in-game purchases as morally questionable if we are talking about paid games here. If you are so eager to share with itch, why not make the currently purchasable content earnable through gameplay and add other things to your presence on itch.io for players to buy if they want to support you further? Like art books, outtakes, scans of development notes (if you still have them), making of videos, and the likes. I’ve even seen paper cut dolls with a game once. The sky is the limit. :)
Personally, I’m not into those kind of games. Kickstarter? Patreon? That kind of game don’t really seem like its suited for itch or its community. Is sounds more like something for an Android app store…
Look: If you have a product that you are sure people will purchase microtransactions for, you should think much bigger and get a (paid) professional to help you with this. If not than you should scrap the whole microtransaction part, just put your game on itch and set it to pay-what-you-want.
In my experience most devs are grateful if they can do as much as build an audience - right now you have not made a first impression when it comes to games. Uploading one of your earlier games for free would help you build an audience here, as well as let you roughly gauge the interest in something of slightly lesser polish. That would be actual information you could work with.