No additional funds for an existing transaction.
A prepaid return label is seller responsbility; you can't bill a buyer for that.
The only correct account procedure would be the refund the original purchase and shipping. it's important to make sure this transaction is refunded and close; otherwise the buyer could open a dispute.
After that, issue a new invoice for the replacement items and send that to the buyer for payment.
Each sale will be separate item and receive approprite seller protection if available.
