From my limited experience of over-chargers the worst offenders have been "commercial" type operations, especially in the US (no offence).
Worst case was US$75 charged, actual postage US$30.
I believe there is a tendency for such operators not simply to cover their costs but to see it as an additional source of income - icing on the cake, as it were.
The seller above never responded to my enquiries. Another "private" operator in the US refunded the excess when I raised the question with him. That's always worth trying.
GG