When adding an eWay payment you manually enter the Credit card number, expiry and cvc details.
The other required billing data to send to eWay is pulled from the customer card. If there is an error with customer details check the customer card details are correct.
Why am I getting an error saying the Customer billing country is invalid?
eWay requires the Billing country to be valid, and will not accept eWay payment.
Example
Valid Country
Australia
Invalid Country
australia