I ALWAYS pack a lightweight stuffpack inside my bag for a few reasons:
If you're ever forced to gate check your bag, you can pull out all your important and valuable items and shove them inside your backpack and stick it under the seat in front of you
Some airlines allow 1 carryon item only, some allow 1 carryon plus a 'personal' item, so having the extra bag along allows you to basically have a second carryon - I usually shove all my valuables in there plus anything I'm going to need on the plane (snacks, pillow, sweater, gum, etc)
They are great for use as a daypack
You can use it as a laundry bag
Since I'm slightly obsessed with lightweight gear I am only including stuffpacks under 200g:
Sea to Summit Ultra-Sil® Dry Day Pack:
This was at the top of my list for a day pack, due to it's light weight & compactness.
Pros: Waterproof, bungy cord system to strap stuff to, weight
Cons: Bag access, non-breathable straps, no exterior pocket, no water bottle pocket
Colors: Black, black & blue, black & lime
Size: 9.45" x 11.81" x 18.11" or 24 cm x 30 cm x 46 cm
Osprey Ultralight Stuffpack:
This is the one I wound up purchasing and I love it! It easily stuffs down into a super tiny pouch that weighs next to nothing. Hardly takes up any room at all in my bag. It seems to be fairly water resistant. It feels a tiny bit flimsy but I don't think it actually is - it just feels that way due to the lightweight materials. I definitely overfilled it on my last trip and it held up just fine.
Pros: Exterior water bottle pocket, small pocket on top that's handy for things like car keys or other small items, water resistant, breathable straps, stuffs down super small
Cons: Not waterproof
Colors: Shadow grey, poppy red, electric lime, tropic teal
Size: 17" H X 9" W X 7" D
Matador Daylite 16: I considered buying this one because it is made with waterproof material and water resistant zippers, but it was sold out. It's fully waterproof big brother looks interesting but also weighs in quite a bit heavier than the rest of these bags.
Pros: Two compartments, two side water bottle pockets, breathable straps, waterproof material & water resistant zippers.
Cons: Weighs more than the Osprey and the Sea to Summit, currently (Jun 19, 2017) sold out
Size: not specified
Weighs in at 116g/4.1oz
This appears to be the waterproof version of their LT pack. Somehow they managed to cut the weight by more than half that of the LT.
Pros: Waterproof, bungy cord system to attach stuff to
Cons: No exterior pockets, no water bottle pocket
Colors: Black, Pewter/Lemongrass
Size: 17” x 9” x 7” | 43 cm x 22 cm x 17 cm
This one looks like a decent and well priced pack, but it weighs quite a bit more than the rest of the bags on my list
Pros: Exterior zip pocket, price
Cons: No side water bottle pocket, weight
Colors: Blue Sea, Black, Red Fire
Size: 11" x 18" x 4.5" | 28 cm x 46 cm x 12 cm
I don't know what to tell you if you're ordering from Canada, their pricing system is not fair. I've ordered from amazon.com and had things shipped to friends in the States or if the item ships to Canada sometimes it's a bit cheaper.