Should I put a note on all listings that Canada Post has made a substantial increase so buyers will know, or will they just not bother to understand?
No. Your customers don’t care what you pay for shipping. They care about what they pay for shipping.
. I have never in 10 years had problems with duties/customs, now Ebay has changed this.
Partly answered elsewhere and the subject of thousands of posts over the past several months.
Briefly, the Global Shipping Program was not introduced for the benefit of buyers. It is meant to encourage those provincial and xenophobic US sellers to open their listings to foreign buyers.
The advantages for US sellers is immunity to INR disputes and protection from negative feedback on the high cost of foreign duty and taxes because the customer pays them upfront instead of on her doorstep.