Restoring purchase of premium features after reinstall
Applications often have a Restore In-App Purchase option in the settings.

Other applications may ask you to "buy again" the item, but you're not actually spending any money again.

It is often mentioned how you can restore your purchases in the application's description on Google Play or in the help category of the application (if any). You can always contact the developer for more information.

(Note that we are talking about Non-consumables purchases, it means purchases that are permanently associated with one's google account. More Information.)