Yes, once you’ve purchased a set that version is yours to use forever. You do not need to pay any additional fees to use these icons. From time to time, we will release versions of the set that have new icons that will require a fee if you want to upgrade.
See the license for details.
No, you can't use or include the icons in items such as website templates (ThemeForest) or print-on demand products (CafePress) that are intended for resale.
Yes, you can use the icons in an unlimited number of commercial or personal projects.