Thanks Ambridge. Sorry to hear that you were fined, but it sounds like you won't be alone if others have been doing the same.

What about this bit .... 

We will be assessing articles processed in a calendar month and invoicing for underpaid postage, plus an administration fee of $1.50 for each underpaid article, in the following month.

Does it mean they're going to monitor all of your parcels for the next month?