Yes, Bileski was a stamp dealer here in Winnipeg, who also had a Post Office box in the USPS installation in Pembina, ND.
Because there was a difference in the price of postage from US to Canada, his American customers could use the US domestic rate to get mail to him at the Pembina address. Also, this allowed him to reply to his American customers and escape the higher rate from Canada to the US by mailing from Pembina.
The labels he produced were stuck onto the letters to US customers to account for a mythical portion of the journey from Winnipeg to Pembina -- which in reality was little more than Bileski driving down to Pembina to check his mail and mail a few letters.
The Pembina address also came in handy when Canada Post
was on strike and mail from the US to Canada was officially embargoed by the USPS (as is typical in a strike situation). By sending mail to him im Pembina, the message still got through despite the fact that there was a mail strike in Canada.
Little more than a gimmick, but very ingenious on his part. What would be interesting would be to find such a label on a letter not originating from Bileski or his Stamp Store, which would show that he couriered for the public at large during a mail strike in Canada. From what I have been able to find out locally he would occasionally do so, despite the fact that such a thing was illegal in those days before private couriers, and the carrying of mail was apparently legally a government monopoly to perform.