I realised that there were certain areas where I could not post anything, but ERNSTC, whose store is Wheresmymojo, could. We thought it was a cookie problem, but after testing it out, the problem doesn’t seem to lie there.
After clicking lots of threads, trying to find the cause, trying to see whether anyone experienced the same problem, I chanced upon a message (or similar, I forgot which! *blush) that said one needs to seek approval to post on certain areas of the forum.
Therefore, if you are experiencing difficulties posting on Zazzle, don’t give up, because it might just be you do not have the approval yet.
Send an email to firstname.lastname@example.org to ask for approval, and the very helpful folks there get back to you very fast =)
Happy posting on Zazzle!