Do you mean the the Canada Post Flat Rate Box, which is a specific prepaid service, that comes with a "free" box and ships for a set price no matter what the weight in the box?
https://www.canadapost.ca/shop/shop.jsf?execution=e1s1
Or do you mean you bought boxes at Staples or ULine or some other shipping company?
If you mean the latter, go ahead and use them.
You just have to scroll a little farther down and enter the dimensions and weight of your filled box in the appropriate place.
Your customer will see the calculated rate to his doorstep.
If you are using the terrible Simplified Sell Your Item Form that newbies are defaulted into, look on the top right of that piece of garbage for a link to the Business/Advanced form which is clearer and gives more options.
https://www.canadapost.ca/cpotools/apps/far/business/findARate?execution=e2s1
The dimensions of my box is 13"x11"x2.75"
No it isn't.
If you use imperial measures you will get incorrect rates.
Canada Post went metric in 1974 and any other system will be GIGO.
for my product it can only fit one pack in one box,
You could use a larger box.
Or you could tape two filled boxes together. Which seems simpler.
The shipping rate for the double order might be lower or might be higher.
Play with some numbers on the website above.
I use 90210 for an easy to remember US zipcode and K1A 0A6 for Canada (It's the House of Commons in Ottawa).
Now go here
https://www.canadapost.ca/cpc/en/business/small-business.page?
and sign up for a Solutions for Small Business number. It's free and gives you access to some discounts.
It also allows you to use the easier (in my opinion) Paypal shipping labels as well as Shippo and SnapShip(Canada Post).
Shipping is the hardest part of selling by mail order.
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