My post office automatically provides shipping. So I ALWAYS have it. IF I don’t use a stamp.
But if it is a card under ~$5 I will do a plain envelope with a sleeve/toploader/bag. That has never failed for me.
But for the higher priced ones I use a small bubble mailer. Or if it is a lot of more expensive cards I wrap them in bubble wrap then syran wrap and shipped in a bubble mailer big enough.
I have never had a mishap.