If I do not receive the refund for the unused postage label I will take the matter further in consideration of the following:
No, the buyer is not entitled to a refund of the £7.19 postage if they received the item on time, as described, and are happy with the purchase. While you made a mistake on the shipping cost, the contract was for the item to be delivered, which was fulfilled.

You are covering the extra costs of your own error. @ GOV.UK +3

• Contract Fulfillment: Because the buyer paid £7.19 and received the item as expected, they have not suffered a financial loss, and the contract is fulfilled.
• Voluntary Coverage: As a seller, you are not required to refund the initial shipping fee if the mistake was only regarding the postage cost, provided the buyer pays the original agreed-upon amount.
• When a Refund is Needed: A refund of postage is only required if the item was faulty, not as described, or if they returned the item under 14-day cancellation