The edd_set_payment_transaction_id() function is used to set the transaction ID related to a specific purchase.
The function takes two parameters, one for the payment ID and one for the transaction ID:
edd_set_payment_transaction_id( $payment_id, $transaction_id );
The transaction ID is passed through a filter called edd_set_payment_transaction_id, which receives two parameters:
Transaction ID tracking was implemented in EDD v2.1.