Set a goal to keep your "Inbox" or "Later" under a certain number of items in Readwise Reader. If you don't, Beeminder will take your money.

Use Readwise Reader like normal. Beeminder will automatically count your Readwise Reader items.
(Details on our blog)

Beeminder will warn you if you're in the danger zone. As long as your dots don't cross the bright red line, your money is safe!