As DG outlined, for the moment anyway, on the AP website the terms & conditions for Click & Send (C&S) indicate that a C&S article with Signature on Delivery (SOD) will have up to $100 compensation for loss or damage.

This area does my head in a bit, so sorry for the long winded post, but there is a bit of information that is relevant - including some inconsistencies.

In relation to Extra Cover, IMO whilst unable to purchase when processing C&S eBay satchels online, according to APs terms & conditions AND the additional services that appear as available when processing a label, you should be able to purchase additional extra cover from PO for an eBay C&S parcel with SOD paid for online. 

I have had no issues being able to purchase additional extra cover for C&S eBay satchels at my PO using the relevant AP form, which is required under the terms and conditions of Extra Cover.

The AP form link is http://www.auspost.com.au/media/documents/registered-post-lodgement-receipt.pdf ( the pdf copy has draft printed on the bottom I do not know why, it is identical to the hard copy version available from POs).

This form is for single articles and includes a tick box for signature items prepared through C&S. There is a bulk lodgement form available as well.  The terms and conditions on the back of the form explicitly provide for extra cover purchase with C&S signature items for values over $100.  

If this is a problem for your post office then you may wish to point out that the product terms & conditions (as per AP website) eBay flat rate satchels & boxes come under the same Terms and Conditions as other C&S articles 

In addition, maybe print out a screen shot  for PO to show them that when you process the satchels even after the new upgrade- extra cover will appear each and every time as an optional extra for an eBay satchel or box.

I also note that if processing parcels via the regular on-line C&S service (not with eBay satchel) whilst you can select extra cover, the amount of Extra Cover payable is calculated on the entire value of goods being processed and includes charges for the first $100, which IMO is contrary to APs published terms & conditions .

Hope all that makes sense.