We recommend that you keep your artwork's Certificate of Authenticity (COA) and only send us the piece that you are framing. That said, we can absolutely create a pocket on the back of your frame to hold your COA.
All you need to do is request that a pocket for your COA be created on the back of your frame in the Special Instructions box at checkout. That way, when your frame arrives back at your home, you'll have a ready-made solution to keep your certificate with your frame.
If that works for you, you can start framing here.