Proof of business license for sole proprietor in California

I'm a freelancer in California, doing business under my own name, no DBA. I company I work for needs proof of a business license.

Thay said this,

"If you do not have a business license, you can send over any other documentation that shows you are operating as an independent business."

They have my insurance policies, I figured that would be enough to show I am operating as a business. What would satisfy this requirement?