For international mail, either incoming or outgoing, there is usually a row of fluorescent bars on the lower back of the envelope, (for machinable mail), along with a two letter month indicator, the date in numerals and a letter signifying the postal district (B= Nova Scotia, E = New Brunswick, V = British Columbia, etc) that is applied at a postal sortation centre.
For non-machinable mail, mail pieces like square envelopes as mentioned, there is usually a place in the sortation center where an employyee or two hand cancel the pieces of mail.
Unfortunately over the last couple years
Canada Post employees do get, I believe, a week off at Christmastime it seems. Nice human touch and all but mail is delayed by an extra week or so, and the extra masses of non-machinable mail letters and packages are sent through uncancelled.
All the above is my opinion only, due to observations from outside the postal system over the last couple years. Perhaps wrong in some cases.
A theory for the reason of payment of non-cancelled stamps is the increase in package rates lke Expedited parcels a few years ago paying for the losses incurred by the resale of non-cancelled stamps. Perhaps not.