Apple manages transactions of purchases.
Apple allows you to purchase many times the applications and the non-consumable in-app*.
If you want to restore your purchases after you install the application again or you reset, buy back the in-app and you pay nothing.
However, remember that in-app must have been bought with the same account of the previous purchase.
(*) The non-consumable in-app
Are the packages that are purchased one-off and will never expire. Remove the application are Advertising, Activate Zapping, the Multiple Playlists and Copy and Paste Playlist.