Genuine customers if they didn't get the keys or have problems with your app would contact you. I do get some who write to me in their native language.
But there are those who probably never intended to pay despite needing the services of your apps. Perhaps they just pay it to get an extended uninterrupted period to get their one off work done then ask for the charge back with some lame excuse to get their money's back.
I do have people who use my app trial, ask for this and that which i fix or add but never buy the app. It's probably just part and parcel of selling online.
Guess the key might be to get a payment processor who doesn't charge a fee or a low fee for charge back and vet the customers through a blacklist database. Can't suggest any at the moment.