This is in England

This one’s hopefully a small one. I ordered a product from a well known online retailer. Basically it was the standard edition of what I'd paid £8.50 more to have upgraded. This is important because originally I emailed customer services to suggest they pay the difference back because I was otherwise satisfied. They weren't very helpful and so as there was the 30 day return window I returned the item via post.

The terms and conditions said that if the item was down to error, they would refund the postage as long as the receipt was included. I included this with a letter explaining that they had made a mistake.

I got the refund for the item but not the postage. It was a small amount but I want it back out of principle more than anything. They are now completely ignoring my emails and calls and I feel robbed.

Retailer is WHSmith - don't buy from their online store.

I can't do a charge back because the issue isn't with Royal Mail. Could I use a small claims letter? Not sure because it does not involve the item itself. Their own terms and conditions say they would refund the postage. Surely that should carry some weight?

Thanks in advance to any suggestions!

I know I probably shouldn't have followed this doggey returns procedure and can see online that other people have been scammed the same way but I felt trapped on what to d— I have record and evidence of everything.