I asked a couple of years ago and discovered that buying envelopes was actually a better way to go. I assume the cost of the envelope could be a bit higher than a label and a regular envelope, but not likely by much. Now I simply purchase 500 Double Window T4 Envelopes from Staples Business Advantage for about $150, which works out to about 30 cents a piece. The T-slips fit the envelopes nicely and they look pretty professional. If you don’t need the Double Window then I think you can get the same size for about $126 for 500. The exact size is 5.75" x 9". This size envelope takes a Canada “P” stamp as it is not considered by Canada Post to be oversized.
You might need to login to see their pricing, so I have listed today’s prices.
https://www.eway.ca/Eway/en/Shopping/Product/SMX9529480FSCNL
153.49
https://www.eway.ca/en/Shopping/Product/SMX9524480FSCNL
126.26
https://www.amazon.ca/Supremex-Envelope-9529480-Double-Envelopes/dp/B00LM6139S
82.99 + 13 shipping