Generally speaking it would be Canada Post that would act on the insufficient postage. Normally the item will be returned to you.
If it makes it as far as USPS they don't care at all what the postage is as they get their cut based on the bulk weight of each class of mail delivered to them. (at the end of the year USPS tells CP what they owe and vice-versa, whichever side is short sends payment to the other).
It's a complicated formula that you can check out at the International Postal Union website. They are the governing body that works out all these things between the various national postal services.
The example I run into the most is with payments mailed to Canada from the USA, many people just use a regular domestic stamp, probably 75% make it through just fine, the rest get returned to the sender by USPS to have the additional postage applied.

"What else could I do? I had no trade so I became a peddler" - Lazarus Greenberg 1915

"What else could I do? I had no trade so I became a peddler" - Lazarus Greenberg 1915
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