You've already gotten some good advice here, but I'll add a few things:
1. Is it worth using the labels? It depends. If you're a casual seller just wanting to get rid of a few things around the house, then it will probably be the easiest solution. If you're planning on selling a lot, then a business account with Canada Post is a better option.
2. How much do they cost? They're slightly cheaper than the Canada Post over-the-counter rate that you would play if you brought the item to your post office and had the clerk do the label. The exact cost depends on where the item is being shipped and how bulky it is.
3. Packaging. I don't sell much clothing, but poly bags are pretty popular (although I would probably double bag them just to be sure, especially if they're higher priced). Most of your items would probably fit in the Canada Post small flat rate boxes also, so that might be a good solution for you - and they come with a fixed cost no matter where in Canada they're sent to.
4. Shipping dimensions. You need to know this. If you get this wrong, it comes out of your own pocket (or makes the item harder to sell if you overestimate by too much). Figure it out and get rid of any mention of approximate costs. When you enter an amount, you're expected to honor it even if it costs you much more to actually ship it.
5. You didn't ask this, but don't use stock photographs of your items. Take pictures of the ACTUAL item. This is very important, especially for pre-owned clothing. Make good and sure you include actual measurements as well as describe in detail any and all flaws.
