You don't need the Priority Medium Box. If you pack it with your own box, the shipping would be $20.75 retail, $19.71 if you print the label online with tracking.
You don't decide if it is worth shipping this or not. The buyer decides whether they are willing to pay up the more expensive shipping to buy.
I always only quote my Canada buyer the lowest Priority International cost + handling + insurance. They decide if they want to pay up the shipping cost to buy.
I just have a Canada buyer, bought a pair of shoes from me 3 lbs. She didn't even ask for shipping quote before bidding. When I send her the invoice for $25.00 Priority shipping including handling & insurance, she just pay up, no questions ask.