So I'm outside USA , and I want to buy the kingdom rush frontiers IAP. Can I do this safely or do I risk getting my account disabled because i use to buy IAP from USA Apple ID outside USA ?
The answer is no.
As long as your account is registered in U.S., and has an active U.S. credit card linked to it, you can make purchase wherever and whenever you want with that account.
I should add though, you should have your account transferred to the country you live. If you're traveling, it's fine.