So, for the future painting supports I am leaning towards choosing 24x36" size as my largest painting size. This size painting will be the largest size (packaged) that I can safely ship via regular shipping companies or Canada Post, and the cost will be less than $100 to ship to US. Since a large majority of purchases are from USA for my art, I am already familiar with the cost, delivery times and tracking of packages, but I have not shipped any paintings this large before from Canada! I made a mistake of simply scaling the cost to a larger painting (which does work, but up to a 24x36" size paintings).
I am including a description of shipping method for my paintings. I have modified it a bit to protect the edges even better (I used a batting all around the corners on the 2x5' painting) since I wrote about this particular shipment in the pictures below.
I found really awesome instructions to ship internationally (or in general) on the website above. Below you will find my version of packing recommended by Laura.
The first part was to wrap the painting in something waterproof - in the picture I used black garbage bags and then packing tape around it. Nowadays I use 4 mil bags I regularly store my paintings in.
Afterwards, I cut the one inch thick insulation styrofoam (bought at Home Depot) to size and again used the packing tape. I repeated the process for both paintings (one was 12"X12" and other was 12"X48"). For larger paintings I use thicker insulation foam (2 in).
I put both paintings on top of each other with the smaller one in the middle of the bigger one. I then cut the cardboard to size and wrapped the cardboard around them both. I filled the air gaps on both sides of the small painting with cheap pillows for extra cushion.
This is a finished package with two paintings in it. It arrived safely to its destination in US. I used Canadian Post to ship. The cost to ship the package was $67, not including the packing materials. This package was about 10"x50"x14".