In my listings the parcel post with signature is not working, but when I chose parcel post without signature it was fine, as was express parcel.  So I listed everything as an ordinary parcel and will pay the $3 from my own pocket.  Spent most of the day yesterday trying to work it out. 

Having the same issue on all listings that require calculated postage.

Just called eBay Concierge and they are apparently aware of the issue and said they would get back to me when the problem was fixed. I won't hold my breath, but will certainly keep bugging them until it is fixed.

They suggested adding a different courier option as a work around as she seemed to imply Australia Post Signature was the issue.

Curious to know if others get the same issue with other postage options?

From what I can see this issue is something that is specifically affecting all calculated postage listings that use the “Australia Post Parcel Post Parcel + Signature” postage service.

It is clearly a system bug/defect that eBay will need to resolve with a technical fix.

You should be able to work around the issue by editing your listings to change the postage service to the “Australia Post Parcel Post Parcel” service and then adding the extra approx. $3 in the 'Handling Cost' field to cover the additional amount required for Signature on Delivery service. Once you've changed the postage service in your listing as above go back into the listing and you should see that postage amount is now calculating correctly for your item.

This workaround has temporarily resolved the issue for me until eBay get around to implementing a proper fix (it will work where you are only offering a single postage service but doesn't help if you are wanting to offer your buyers a choice of multiple services).

Hope this can be of help to anyone else affected by this issue.