How do we add just one more Mail Suite user without paying for them all again?

How do we add just one more Mail Suite user without paying for them all again?

In Zoho Mail Suite I am trying to add one user. When I go to the Control Panel and click on Subscription, it shows the current 30 total users we have (20 paid and 10 free). How do I just add only one more? If I go to 31 in the pick list then it wants to charge me $105. instead of just the extra $5. for the one user. Please advise this is causing issues for us.