If the customer paid for express post and you sent by regular mail ebay will bill you $4.

Ebay  customer  service is the worst ! It will take you one thousand year to search the chat button, once found, you need to type "an essay" before an agent can chat with you online. Agents have limited authority and you won't be able to escalate the case to a customer service manager as the agents claimed they are the ones who have the full authority to handle any complaints.  Go somewhere else with better customer service support.