Free Plan: 10 Users... Paid Plan: 1 User?

Free Plan: 10 Users... Paid Plan: 1 User?

Hello everybody.
 
I just starting using Zoho Mail yesterday with the Standard 10GB plan. I had a bit of trouble with the DNS settings at first (changing the MX records) but finally managed to get it working.
 
Now, I was about to add a New User, but I find I have to pay an extra amount (another $24 per year) for that user. In the "Max number of users" it said "No upper limit" so I thought this included an unlimited number of users (for that price). I checked and yes, the price says $2.5 per user per month, but if the "Free and Ad-free" plan allows for 10 users, how can the "Standard 10GB" paid plan allow only for one?
 
I think I might have made the wrong choice of Plan. Can I change my plan and get a refund?
 
Thank you