I found two ways for in-App Purchase. Which one is best for using and why?
Urban Airship uses Apple's API's under the hood, they just provide a wrapper that's easier to manage.
I would take a look at the docs at developer.apple.com in the in-app purchase section and see if its worth paying urban airship the extra couple bucks. It's pretty easy to implement simple in-app purchases (ie unlocks with content already inside the app) but if you are going to be doing a lot of them (expecially server side delivered in-app purchases) you might go urban airship since they host content for you also.