Let It Go is obviously the song from Frozen which is copyrighted by Disney who in the past has been quite strict about use of copyrighted property (they have loosened up recently). It is possible that Zazzle is erring on the side of caution given Disney's reputation for lawsuits.

As the others say, don't mess with Disney. You can't copy any well-known phrases or images, but you'll find that there's some flexibility there. I have several stores on Zazzle. One uses public domain images from a single source--a government agency. However, there's been some litigation from some of the people in the photos. As a result, most of my items (using maybe 75 images) have been approved, but 5 or 6 have been rejected.

Yes. "Let It Go" is the title of a copyrighted song, owned by the Disney corporation. Adding in a snowflake just gives them a stronger case for copyright infringement.

I have several stores on Zazzle

I tried to make a shirt that said "Let it Go" with a snowflake and I got back a response from Zazzle that I could not be processed because of copyright material? Are those words copyrighted? I like using the website, but didn't know it was so strict?...