Hi Stephanie :)
It sounds like you're getting frustrated, and it's totally understandable - especially after going to a gazillion outlets and basically getting no information. Sorry about that!
Within Canada you would be able to use Oversized Letter:
Envelopes must be no thicker than 2cm or .75 inches.
maximum measurements - 38 X 27 X 2cm
Here are the prices:
www.canadapost.ca/tools/pg/prices/Canada.pdf
Outside of Canada (US and International) your packages would probably be able to go by Light Packet if they stay within these measurements:
Maximum dimensions: 380 mm x 270 mm x 20 mm
and Minimum dimensions: 140 mm x 90 mm x 0.18 mm.
I actually don't know if Etsy labels are available for Oversize Letter within Canada, as I don't use them.
But they do offer Light Packet labels.
In order to stay within the measurements above, you might have to stay away from bubble envelopes, unless your packaging inside is very thin (tissue paper, etc). Best thing to do is get an envelope, package an item and take it to the PO (Gah! Sorry to send you back there again!) and ask to see if it will fit through their slot for Light Packet (which is not-so-affectionately called the 'slot of doom' by most sellers).
If the bubble mailers do fit, then it is completely up to you if you use them or the plastic ones - whichever you like/think will work best for your products.
Custom labels are really easy to fill out, I grab a bunch at a time and fill them out at home before going to the PO to save time while there. You should be able to ask for a few, so you can see what they look like.
But, if you use Etsy labels, I believe the customs information is printed with/on the label which takes care of that extra step for you. (Again, I don't use the labels, but pretty sure that's how it works.)
You will definitely need a scale at home if you are going to use Etsy labels, but they can be purchased for pretty good prices.
An excellent information thread on all things Canada Post:
https://www.etsy.com/teams/7654/etsy-international/discuss/6779956/page/1(Note that the thread was started in 2011, so the prices, etc that you see at the beginning are not up-to-date. But you can join that team and post your questions to get all the answers.)
Good luck! Let us know when you're open so we can check out your shop!
~Holly
xo