How Do Customs Charges Work, Buying From Overseas To The Usa?

No, non US post offices hold a different view on this. So, they are getting different information from the US government.

That is an assumption, not a fact.

[removed otiose recap of info]

On Friday I received an e-mail from Canada Post that since the USA has suspended its duty-free "de minimis" threshold of up to US$800 all "postal shipments up to US$800 in value to the U.S. now require duties to be assessed and prepaid before your package crosses the border". If this is not done the item will not be accepted by Canada Post. Pretty much kills sending anything to the US from Canada that has any value.

But this does NOT apply to philatelic stamps and covers because there are NO tariffs on these items and the HTSUS which Canada should be using reflects this.

Understood. Tell this to the local postal clerk. I've already had one rejected because they wouldn't accept my argument that there is no tariff due on postage stamps and local stamp dealers are specifically stating on their auction sites that they can no longer send to the USA.

I think the other problem is that once the item crosses the border and doesn't have a "tariff paid" receipt on it, the item gets rejected and returned to Canada. My local postal clerk has told me that in the past they used to get an occasional item returned for some reason, whereas now it is a regular occurrence and therefore, they are taking a hard line and basically not accepting any item that does not have a prepayment of the tariffs so that they can attach the appropriate form instead of dealing with rejected mail. It is a mess.