Hi danclark,
Welcome to our Mattress Forum! 
There have been many changes to the Tariff Schedule over the last few years, as well as the changes in NAFTA. In general, Mattresses are considered under chapter 94 of HS 9404.21, which was at 3% in 2020. A revised 2022 of Harmonized Tariff Schedule of the United States Revision 11 here reveals no changes for Canadian-made mattresses with the same 3% as before – This would be calculated based on price without shipping, of the mattress. For the latest updates, you can always visit the US CBP (Customs and Border Protection) Attached is the pdf directly from their website.
https://uploads.mattressunderground.com/outside/66/USCBP2022Ch94ImportingBedding&Mattressesshttps://uploads.mattressunderground.com/outside/0https://uploads.mattressunderground.com/outside/1https://uploads.mattressunderground.com/outside/2
The minimum threshold for import tax is $800 USD. Articles valued below that are not subject to duty. Here is a quick overview of the importing process
For those interested in importing a mattress from the US to Canada there are some good posts from MFC and Mattrebuild from January 2022 in terms of customs duties in this thread on US mattresses for Canadian Shoppers
I hope this is helpful
Phoenix