The idea of click and send satchels is to give you seller protection against item not received cases.

You must take the package into the PO and they must scan it on lodgement and give you a receipt as that is you proof of postage.

If the PO staff tell you any different tell them they are obliged to accept and lodge the package over the counter.

Tell them (while you are there) to call the number on the satchel to verify they MUST accept and scan on lodgement as that is what the C&S postage was created for.