Let’s say someone from the US is mailing me an item worth US$348 and the sender declares it as such on the declaration slip. It is not marked as a gift.
How much is taxes/tariffs/whatever will I be charged upon receipt here in Canada?
Why would I be charged? Pardon my ignorance, but does NAFTA come into play at all in this scenario? Where is the free trade? 
It depends on the actual contents.
We often have things sent to us from the U.s. and each is a different case, the contents of the package are the determining thing.
Call the post office, Canadian, and ask, they should be able to tell you, though they are sometimes mistaken.
We sometimes dispute with them about which articles have duty and tariffs, there are always some things that fall into a grey area and require a ruling.
Without knowing the contents of the package there is no way to answer this question.
What, you believed anything that Brian Mulroney said?
You may or may not be charged duty and taxes. Depends what’s in the parcel. Depends also if it just goes through unchecked (possible, but not likely).
Handmade all wool rug from Turkey? 14% duty, plus GST and PST.
Used auto parts? No duty, but GST/PST may apply.
Books? No duty, but you may have to pay PST. And, if your parcel is stopped and examined, there’s a lovely CDN $5 service charge (whether or not any duty or taxes are charged):
http://www.cbsa-asfc.gc.ca/import/courier/postal/ind_dutiable-e.html
It’s a Coach handbag with an MSRP of US$348. It is a US brand, AFAIK, and I don’t know if the person sending it to me will be asked where the product is made. It is made in China, but I don’t know if she’ll even note that.
It is coming from Florida and I am in Toronto, Canada.
Thanks for all your help. 
One hint. Make sure they MAIL it (as in the US Postal Service). Tell them NOT to send it by UPS. I don’t recall exactly what the current brokerage fee that UPS charges is, but I do know that it is a ---- of a lot more than the C$5 charged by Canada Post.
I don’t think the handbag would fall under any of the special categories, so as others have said, you’d get to pay GST + PST + C$5 to get Canada Post to give it to you.