"Did Canada Post insist you pay the postage a second time or did the buyer have to pay extra at their post office in order to receive the items? "
In the few instances where postage was insufficient, items were returned so that the additional postage can be affixed to the envelope to make up the correct rate. No penalty or extra cost were involved
The problem is time. In two instances, letters were returned to me four weeks after being sent.
Canada Post added 400 employees last year to their "revenue protection" program - spot checking outgoing mail to make sure folks pay the correct postage. Still, millions of pieces of underfranked mail go undetected.