Will brownies remain fresh if sent by post?

Brownies should stay excellent for several days at room temperature. For several more days they will have diminishing quality, but still remain tasty enough to be worth eating. After about a week to ten days, they're usually more stale than most people will consider worth eating. Of course the difference between "worth eating" and "not worth eating" is subject to personal preference. Any brownies that you won't consume within the "still worth eating" window, you can store in the freezer for several months. Thaw at room temperature, or by gently heating in the microwave.

Usually you can get one day or two day shipping, which would arrive within that "still delicious" window. In COVID times, shipping services are having lots of delays, even if you pay a premium for fast shipping. So it's difficult to predict whether your brownies would arrive while still delicious, or even still good to eat. I suggest having your friend weigh a batch of brownies (don't forget to add the weight of the packaging). Use the shipping calculator on the website for Canada Post (or whatever shipping service you want to use) to calculate how much that package will cost to ship. Then decide whether you're willing to gamble that amount of money. You could have her send you just a couple of brownies as a test.

To ship them, your friend should make sure the brownies are completely cool before packing them. (If they're still warm, they will release steam inside the packaging, which will make them soggy.) Wrap them tightly in plastic wrap and place inside a plastic bag with a zipper. Put the bag in a box. If there's loose space in the box, fill it with crumpled paper or other packing material to stop the brownies from moving around and getting crumbled during transit. Particularly dense brownies might do alright in a padded envelope, if you want to save costs and are willing to risk them getting a bit smooshed.