On the Buy Credits page scroll to the Auto Recharge card:

1. Switch the Status toggle on.
2. Set When balance is below ($) – the threshold that triggers a top-up (default 10).
3. Pick the Refill amount – $50, $75, $100, $250, $350 or $500.
4. Click Save settings ("Changes take effect immediately").

Whenever Sprwt is about to send messages (when you open the confirmation step on Compose and each time the sending, recurring and abandoned-cart jobs run) it checks the balance; if it is below the threshold, the refill amount plus a 3.5% processing fee is charged to your card on file and the refill amount is credited, logged as Auto Refill. Nothing is credited if the charge fails.