Sounds like a really crappy experience. You might want to contact Sixt's customer support team and point them to this post and see if they would be willing to refund your $23 at least. Then at least you could post a follow-up here to show that they have a commitment to customer service at the corporate level. It would be a win-win for both of you and maybe they'll even try to address the issue with East Coast, but that may be a bit too much to hope for.

You might find this informative:

http://www.eastcoastcarrentals.com.a...als-down-under