I am being overcharged just like you.  I had one parcel to travel only 10 kms, and while the buyer got a discount on the postage, to print the label it would cost me $5 more than the buyer paid.  Ebay offered to refund me the difference if I printed the label, I declined and paid over the counter at the Post Office, and while still out of pocket over what the buyer paid, I was only out of pocked 20 cents, not $5.  I since changed my listings to "flat rate" not "calculated" and enter what Australia Post quotes online (you can enter, size and weight, own packaging etc and get this from their website) and that amount is always accurate to what they charge over the counter. However the quotes for printing a label are still too high, so no choice but continue to pay over the counter.  The latest one which posts for $14.10 (under 500g with siganture on delivery) Ebay and Australia Post quote $19.93 to print a label and state that they are giving a 25% discount. Are they kidding??