Are you seriously telling me that Australia Post has some way of checking when the satchels were bought, and more to the point...are they really checking them?

Come on, Auspost is not that clever!

Keep using them, Prepaid is prepaid. You have already paid for the service for Auspost to deliver them. It does not state anywhere on the satchel that they have to be used within a certain time frame. As far as I can see if you use the same satchel 10 years down the track they should still have to honour it.

They have kept your money for 10 years and collected free interest that that money as well.