I think the answer is simple, it is the sellers responsibility to deliver the item for the amount that was advertised.
Perhaps sending a scan of the package showing the postage due would help.

I think many times we are concerned more with preserving our feedback than posting our real feelings.
You have to decide what is more important to you.
Remember, we would all benefit more from the feedback forum if it were used properly.
At present I have been lucky enough to have preserved my feedback rating. I did have one negative posted, I fought it and had it removed, no easy task.