When I started selling a whole 7 months ago
🙂 I used to write all the information on the back of my cards. Specifically, Title, date created, medium(s) used as well as type of paper, and signature. Then I was reminded about the importance of the copyright emblem with artists full name and year. Then a card I bought came with a COA, and I thought that was great. So ....
These days, I print a separate COA that has a small image of the work, its title, date created, materials used (which includes the type of paper as well as the mediums), the copyright symbol ("c" inside a circle) with my full name, then a place for my signature and the way I sign my work. Finally, I add my website address.
Sounds like a lot, but I've managed to fit it all on an ACEO sized form that I put inside a penny sleeve with the card (or separate sleeve if they want the extra border around the ACEO image left for easier framing.)
Here's one I did for a recent ACEO:

I also write with micron pen on the back of the actual card its title, date created and the copyright stuff.
maggieB