I've come across it in another context - opening electronic access to
print subscriptions, via the publisher.  The publisher says blandly "To
do this, we will need your customer number.  You will find your customer
number on the mailing label".

This is another instance of the ineptitude of publishers when it comes
to practicalities.  Obviously they have an image of all their
subscribers as individuals, coming down stairs to scoop up the mail from
the doormat and read it over their coffee and boiled egg.

Cheers anyway,
Lesley Tweddle
Serials Department, American University in Cairo Library
<ltweddle@aucegypt.edu>