Hi, Paddy, this was an issue sometime back where C&S generates a tracking number which is automatically uploaded to ebay item, when the second number is generated when you pay at the PO this number is not uploaded,

and this was the number that was being scanned in (as the PO staff were sticking the postage label over the C&S barcode) and was not linked to C&S and Ebay.
What they have done is to process it as Parcel Post and not under C&S section, where, when the C&S barcode
is scanned in it will then generate a matching barcode/tracking number on your postage sticker.
Thus avoiding potential problems.

The 3kg C&S satchels are $11.70 and yes it is cheaper to send within your own state using your own packaging.
The satchels & boxes are a flat rate Australia wide, and is usually cheaper for most destinations.

Barrie