SMS is prepaid. You add money to an SMS Wallet (Buy Credits page) and every message segment is deducted at the per-message rate Sprwt has set for your account (the Compose page shows it as "Cost: $X/per SMS"). A long message counts as several segments, so it costs several times the rate; texts you receive are also charged one message each. If a message fails or is undelivered the charge is refunded automatically, and if the carrier reports a different segment count than estimated the difference is corrected. Your balance is shown on Compose, Chat and Buy Credits, and every change is logged with a reason: Bought By Admin, Added By Super Admin, Auto Refill, Received SMS, Sent SMS or Sent SMS Refund.