When you receive a delivery receipt with a FAILED status for an AlimTalk or a FriendTalk message, then you will not be charged for the FAILED message(s).
For a KakaoTalk chat, it depends if have previously replied successfully. If you have replied successfully before then a session fee will be charged regardless if the subsequent messages FAILED to deliver. If problem persists, you can contact support@sinch.com for help.
You can learn more about receiving delivery receipts on the Sinch Developer Documentation site. There, you'll find comprehensive documentation that illustrates how Sinch supports KakaoTalk through the Conversation API.
