Yes. Your WebCatalog subscription includes access to Singlebox, so you don't need a separate subscription.
Sign in to Singlebox with the same WebCatalog account that has your subscription, and your paid features will be available automatically.
Singlebox is a separate app from WebCatalog, but the two products share the same account and subscription system. You can use Singlebox, WebCatalog, or both with the same subscription.
Learn more about Singlebox at singlebox.app.