It is you as the seller to get the postage correct. It is not the buyers fault if you offer options, but have the cost wrong.
You should refund the buyer the £1.50, as it was your error.
I would point out that if buyer had opened a NOT RECEIVED CASE, you would be more than £1.50 out of pocket, and as delivery was late, buyer could open a NOT AS DESCRIBED case, even now, and that would be even more expensive for you.