Once the program is launched and once you are opted-in, you will be able to override eBay’s Estimated Delivery Dates with your own timeframes, provided by your carriers. For example, if you know that you can deliver an item in 3 days and are able to provide tracking, you will be able to update this in your Rate Tables, which we have updated to make this possible."

(^that's from eBay's FAQs)

 

Rate tables provide the option to set up delivery costs by region, and I suspect given the above, they may also provide the option to set up expected delivery times by region.

 

That being said, I am unlikely to participate, for I have no interest in providing guarantees on behalf of Australia Post, based on either my experience or AP's stated timeframes, and them compensating buyers out of my pocket if it gets there the day after. I will continue to offer express for those who want faster deliveries, and will fix problems to the best of my ability if they arise, but I won't go out of my way to create problems where currently none exist.