New Zealand Issues On Fragments... Ctos?

Found a stack of these things in a box lot. Some if not all appear to be CTOs based on the "PHILATELIC BUREAU" cancels, but they are on irregularly cut parcel/envelope pieces.

1. Was this normal practice for the period? Why are they on fragments?

2. What is the meaning of the purple handstamp "THESE STAMPS REPRESENT THE POSTAGE DUE ON THIS LETTER" and why does the first one have a blank space where the word "DUE" should be?

3. Do they have any value? I assume the last two have at least some value even as CTOs based on the sheetlet catalog values in Scott (US$13 mint, US$22.50 used, each) and Gibbons (23 GBP mint, 40 GBP used, per pair).

If I remember correctly this was the way that NZ Post used stamps that were used to pay for postage on orders from them.

Years ago, when the "PTT" was still the postal service in the Netherlands, this is much like the Philatelic Service used to mail collectors' orders also. They cancelled the franking and included them with the order. I do not believe these to be "cancelled to order" but actually used!

Peter