Her statement about 'all sellers do this' is just plain rubbish - I only ever charge actual postage costs, and all items I've recently purchased have only charged the actual cost as well.

However - it is postage and handling costs, not just the cost of the stamp. What's to say she doesn't employ someone purely to do up and post her parcels. That would be a legit handling charge, woudn't it?

When you purchased the item, surely you knew the cost of the postage then. When I buy something, I look at what it costs me in total. If you are happy with that price, then what does it matter what the postage charged was, and if you were unhappy you shouldnt have purchased in the first place.

Leave her low postage costs DSR if you feel it justified, and as another post mentioned, 'okay' for feedback comment is probably something along the lines of what I would do...more so for her lack of respect in her email to you than anything else.