Excursion: Postage Due Cancels

Envelope received from an Ebay purchase

cancelled with "Postage Due Unit" CDSs

This is obviously not a revenue document.  But it was used to ship 1898 revenue stamps.  I had saved it when it came in last year since I thought it was interesting.  I've never received anything like this in the mail, and the careful handstamp cancelling of the 1960s vintage commemorative postage stamps with the Postage Due Unit CDS stamp makes for an interesting curiousity.