Is it even legal?

[boatie_au]

I have sold a luxury bag via ebay and during the postage process I have measured only size of the bag box and weighted then issued an ebay postage label. After arriving the post office, I have purchased a post office box to further protect the parcel. So the pracel box postage price was $23 and buyer received the item all good happy days.

But over a month later I have been issued with a huge $129 fee for "postage adjustment fees" since the actual postage dimension was slightly bigger (less weight). I just can't figure this out on top of my head how could this even happy, could this be prevented via posting stage I would happy to add bit extra fees to cover extra size of the parcel at the site but not getting hit by this huge totally unexpected bill after a month later. 

Not sure what they doing to protect the seller's rights seems non 

[shoppingbag*]

You should have taken it back home, cancelled the first label, re-measured and weighed the item and printed another label.  There are warnings when you print the label to check the size and weight.  Even if the parcel is 2mm over in size you can be fined heavily.

[smoothmotiontech]

There should be no "heavy fines" for 2mm over.
In fact they don't "fine" you at all. Any additional charges are related to "what it would have been if calculated correctly" plus potentially a small processing fee.  So no, 2mm over will NOT trigger "heavy fines". 

They round up to the nearest cm so yes 2mm over could round up to another cm, and that 1cm more could tip it into a new price bracket if it goes over a certain cubic size. But that's just a cost recouping, "heavy fines" implies there is a crime here, which is not the case. It's just you being charged what you should have been charged if you had put the measurements in right.

Now does this $129 or whatever it was from the OP sound reasonable? Probably not. But without knowing what the parcel size and weight they put into the system, what the actual parcel size and weight was, if it went from a flat rate to a calculated rate, and where it was going from and to, we can't know. On the surface it sounds high and they should challenge it, because the scanning systems DO make mistakes sometimes. But either way it's not from "heavy fines"