Shipping

Not to tell you how to personally do your business selling things, but why not just go with the higher estimate by using your Canada Post web site, for the postal rates, and then be done with it.
Your gonna be damm close, and won't be breaking the buyers piggy bank, or bank account.
Sounds like you have done enough selling in your career to have a good ball park figure on the weight of the part(s) that are in the box.
Go by your gut feeling, and past experience.

Your posting has morphed from, asking why won't sellers ship, to your having problems with buyers, paying you.