As a new seller you should also know that your customer's cleared payment will be Held against your good customer service.
So be sure to ship as soon as the payments clear. If you use tracked shipping that will help remove the Hold faster.
You can use the Held funds to print a shipping label through Paypal. Get a Solutions for Small Business number from Canada Post and you may also get a discount on the label.
https://www.canadapost.ca/cpc/en/business/small-business.page?
It looks like you have already found the Canada Post shipping rates site but here it is for other new sellers who may be reading.
https://www.canadapost.ca/cpotools/apps/far/business/findARate?execution=e1s1
It's worth bookmarking.
BTW, you cannot change the shipping cost after the item has sold, which makes this
" there is additional charge for a new Canada Post bag or box. " questionable.
Canada Post has prepaid shipping boxes which may actually be cheaper than your Calculated Shipping cost, but there are no discounts available on them.
The prepaid boxes can be purchased in bulk from Canada Post online, or one at a time at the PO.
