I wouldn't even think of using the Post Office for bulk submissions to PMG or PCGS. I use FedEx without any exception whatsoever. It's not cheap ($50-$100 depending on the package weight) but the quality of the tracking and the speed of delivery are mesmerizing. A package sent from Ottawa before 4:00 pm one day will arrive at PMG in Florida before noon the following day, and vice versa. My customs declaration always reads "Printed documents being sent to PMG for certification, then returned to sender", or words to that effect. Fedex does not deliver to PO Boxes though.
Last October, I lost a $300 banknote that I sent to the UK by Registered Post. It was tracked through Montreal and to Heathrow Airport in the UK but then it disappeared. I'm still waiting for some sort of minimal compensation from Canada Post and that's after four months and two online complaints. I don't blame Canada Post in this case as the item did arrive in the UK although it never got to its proper destination.
I've used FedEx for overseas deliveries too. A banknote sent to INDIA arrived TWO DAYS LATER, as did a banknote sent to QATAR. The cost in each case was about $110 but it's worth it if only for the peace of mind. Registered Post would have taken three weeks or longer, and the item may have been lost or stolen along the way.
FedEx Forever is my motto!
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" Buy the very best notes that you can afford and keep them for at least 10 years. " (Richard D. Lockwood, private communication, 1978).