You can find the ARN in the Customer Area > Transactions > Payments > [PSP Reference] > ARN Number: [ARN Number]
In credit card transactions, the ARN is a unique number assigned to a transaction when it goes from the merchant's bank (acquiring Bank) through to the card scheme at the cardholder's bank (Issuer).
The ARN can be used to track a payment or refund. The shopper should contact their bank, who can use the ARN to track the refund.