The transaction ID is a merchant generated reference to a transaction. Typically this is also the same as the order ID from your order management system or just an incremental unique identifier for each request / transaction sent to Peach Payments for processing.
The transaction ID is supplied by the merchant in the API request in the parameter merchantTransactionId.
It is not generated by the Peach Payments' system but in fact is an essential part of the transaction request.
It is important to keep the transaction ID unique because our system will reject duplicate transactions i.e. that have the same transaction ID and the same amount.