No. If a message is failed, the credit cost of the failed messages will be added back to your account balance.
Please note that Pony Express charges for the attempt of delivery, so any message marked as 'accepted' will be charged unless it is failed. In some case, messages may be sent from Pony Express but are not received at the end user's handset. This can happen for a number of reasons, including carrier content filtering, availability of the destination phone, etc. Please contact support for detailed troubleshooting steps.