Having brought up the Postage Due issue, I will relate an experience I have now had happen twice for very similar bubble-mailer packages. Both of these were brought in to a Canada Post outlet and I was charged postage for irregular mail. On both of these, I had put my return address seeing as it was going through the hands of a CP employee.
Both of these were returned to me saying "Insufficient Postage" and the last one saying it had to go as a package with a customs declaration which would add much postage. On both of these, I proceeded to remove my return address and put them back in the mail. Because they are destined for the USA, no postage due will be charged and they will be delivered (one was a long time ago and it got there on the second attempt; the other is on its way).
All this to say that one CP/ USPS employee may charge a postage they think is sufficient and, further down the line, another employee who interprets the post office's rules on determining postage may think there needs to be more postage.
Bruno
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Check my Profile page to see my 2023 Goals and my Lists of sets near completion (5 cards or less) or sets getting close (less than 100 cards missing and 75% complete).
https://www.tcdb.com/Forum.cfm/Page/B/ID/0/?MODE=VIEW&ThreadID=25745&C=0