Buyer made an offer plus postage which I accepted but the sale wasn't completed so after 3 days I revised the listing with free postage to get it sold. Then she emailed to say she could now complete at previous agreed terms, would I accept, I did, but forgot to add the postage (£3.95) back on. She paid immediately, now won't respond to my emails explaining my error and requesting postage - I'll have to accept it won't I