We do, for the Tools & Standard Plans.
Tools Plan: Has a limit of 50,000 visits per month. After reaching this limit we'll charge $5 for every 50,000 new visits. You can opt-out from the extra visits packages in your Site Options area. If you do, we'll stop serving our apps once the limit is reached.
Standard Plan: Has a limit of 500,000 visits per month. After reaching this limit we'll use a pro-rata system to charge the visits in excess.
Expand Plan: Has unlimited visits per month, contact us for a quote.