You're getting charged for using purchased assets (images or/and video clips) in your project.
Wave.video stock media library contains free and purchased assets. Purchased assets are high quality and royalty-free footage and images coming directly from leading suppliers of stock assets (Getty and Shutterstock). Users have to purchase these assets to be able to use them in projects.
Users only have to pay for a premium video clip or an image once. After that, they can reuse this purchased asset in their projects an unlimited number of times.