There's nothing to worry about!
When you return an entire order to store within 14 days of your order date, the system automatically refunds your original delivery cost as well to your original payment method, as the till system cannot always include this in your store refund.
The payment method used for the refund in-store will be credited with the refund value within 5 working days.
You'll receive an email to let you know that the postage refund has happened, and it will outline the payment method that went back to (in the example below the customer had paid postage of £2.99 via PayPal). The rest of the refund is listed as 'PrePay' as the information on the refund you received at the till is securely hidden.