externalPaymentId is an ID that the POS provides for two functions.
- For payments, it is an attribute that is added to the payment.
- For payments and manual refunds (Credit) it is also used to prevent duplicate requests from processing. If the device receives a request (Payment or Credit) with the same
externalId, it will fail the transaction. This is to help in the event the POS doesn't receive a callback for some reason and/or tries to process the same transaction but the device has already received and processed the request.