Jetstar make mistake and REFUSE to engage

I flew from Sydney to Melbourne for a conference last week. I selected a carry-on only fare because I was going to a conference and only needed a change of clothes. Took no computer because the presentation was online. At Sydney they weighed the carry-one and handbag; they (apparently) weighed in at 9.5kg even though I'd weighed it at home and it came in well under the 7kg limit. It was very strange and I thought an error had occurred. I was directed to the desk where they asked for my credit card, I handed it over. NO words were exchanged. They charged $60. I thought it weird but there was a long line of people behind me (lots of people were in a similar situation) and I didn't want to make a fuss so just gave them the benefit of the doubt. The next day, I paid an extra $27 for the luggage on the way back. But at Melbourne airport the automatic weighing belt weighed it at 4.5kg, which is more consistent with the weight I had determined. The person weighing the luggage at Sydney had clearly made a mistake. I went to the website when I got home to raise the issue and was met with a response of basically: 'you are wrong and we are right. I complained and they were like 'here's a $50 voucher to go away... I did not want a $50 voucher. I wanted the $87 they took from me in error. They were dismissive and patronising and flat-out refuse to engage! I'm so cranky! They are extorting funds from passengers, and refusing to address the consequences!
I started a #jetstarcomplaints hashtag on twitter. Let's chat there and make them accountable for their disrespectful attitude toward passengers.

UPDATE: Today (22/11/17) Jetstar agreed to refund the money they'd take from me. I was happy about that. I only wanted the money that was unfairly paid and I got that, so I am satisfied with the outcome.