Best way to ship cheap cards in Canada?

Thanks so much for such a detailed quick reply. With what you’ve shared, it does seem like I should be able to use these small bubble mailers for non-standard lettermail. However, I have a couple more questions if you, or even someone else wouldn’t mind.

Can I put stamps on the bubble mailers that are within the dimensions of non-standard lettermail and throw them in the CP mailboxes for pickup, or do I have to take them to a post office?

Also, when I put a toploader in the bubble mailer sideways, it’s a near perfect fit, but with a fair bit of extra room length-wise. If I fold the mailer in half it becomes a true perfect fit and essentially makes it impossible for the toploader to move around at all, but the length of the mailer becomes lower than the minimum requirement of 140mm. Are we just not allowed to ship any items smaller than that? Do I put the folded bubble mailer in a bigger envelope to get around that, or is it just not necessary to fold in half?

Sorry if I’m overthinking all this. I just wanna make sure it’s done right.