The shipping amount the customer paid is fixed, and doesn't change whether your label costs more or less. This is why you have to know your price ranges, because you obviously can't ask the customer for more later. You can add tracking under progress and just fill in the postal service you used.
What you need is to have your own weigh scale, I have a flat digital cooking scale. Weigh a couple of the frames with packaging and figure out what the postal class for that weight is. On Etsy you have a blank for the shipping and then a with another item amount. So one frame is say 10 and then if two frames together cost 15 , then the additional amount would be 5. Some people have flat rate shipping profiles, some have calculated shipping (where the customer adds their address and you have a set size, and it calculates the prices to different places). Once you have an idea of the costs it isn't that hard.
If the costs are very different for the different amounts of frames, you could use variations so the customer buys one or two or five, with a little extra added to cover the gap between postal prices.
Meanwhile Canada Post has a business shipping discount you can sign up for, and it also has some flat rate boxes. You might want to explore what works best for your items. Each shipping class has size and weight limits. Canada Post has ways to look up rates, if they ever finish updating their site.
