Thanks heaps, Digital Ghost!

Hopefully, Cat's PJs will log on at some stage and share how her local PO does this (i.e. adds extra cover over $100 to an eBay Flat rate Satchel).

Phorum Junkie's PO seems to be able to as well? Mentioned above, and I remember her also mentioning it a long, long time ago:

"Signature on delivery at $2.95 gives cover up to $100 and you can still purchase extra cover for over that as you always could."