Sounds to me like you need one. I guess it all depends if they consider you as a manufcaturer or not. I would call them and get a definitive answer from someone who knows what they are talking about. Are you a manufacturer? Straight from the website:
How to obtain a garment registration certificate
"Garment manufacturing" means sewing, cutting, making, processing, repairing, finishing, assembling, or otherwise preparing any garment or any article of wearing apparel or accessories designed or intended to be worn by any individual, including, but not limited to, clothing, hats, gloves, handbags, hosiery, ties, scarfs, and belts, for sale or resale by any person or any persons contracting to have those operations performed and other operations and practices in the apparel industry as may be identified in regulations of the Department of Industrial Relations consistent with the purposes of this part.