Humm, can't say I've had one and my Courier has been coming through fine.
Did those come through as part of your renewal, with a specific Courier or just 'out of the blue'?
With the demise of the paper tax disc how about selling either one-off or annual TSSC 'tax discs' to fill that other-wise empty space on the windscreen that just doesn't look right being empty?
Could be just an alternative to the woven badges or something a little flasher like these guys do - http://www.creativetaxdiscs.co.uk/page46.html