I love your cards! Nice shop too.
Not sure what you used to package the card, but bubble mailers are lightweight and cost less to ship. I get mine from
http://royalmailers.com. I also put a piece of mat board or foam core board (most lightweight) in with the item to keep it rigid, but that does add to the weight.
I also write "Do Not Bend" on the package.
It might not be worth it to do singles on the less expensive cards. Maybe try a listing that bundles two or three of them together to make a set? That should help with shipping.