The "Inactive customer account" error from FedEx typically indicates that the third-party account you're attempting to bill is currently inactive. This can occur for several reasons, including:

Account Inactivity: The account hasn't been used for an extended period, leading FedEx to deactivate it.

Billing Issues: Outstanding balances or payment issues associated with the account.

Incorrect Account Information: The account details provided (such as the address) don't match FedEx's records.

Recommended Actions:

Verify Account Details: Ensure that all account information, including the account number and associated address, is accurate and matches FedEx's records. 

Contact FedEx Customer Service: Reach out to FedEx to inquire about the status of the third-party account. They can provide specific details regarding any issues and guide you on reactivating the account if necessary. 

By taking these steps, you can identify the root cause of the error and work towards a resolution.