You will get these and low DSRs even if you charge below postage cost. I would not worry about it.

In your reply to the feedback (if you felt to compelled to reply) I would have confined it to something factual along the lines of "Postage charged at actual cost + 50 cents handling charge for packaging materials" or similar. The buyer bought the item at the specified price and then complained about it. There is nothing you can do to avoid these situations unfortunately.