Cannot purchase more mailbox

Cannot purchase more mailbox

Hi guys,

I would like to purchase more mailbox for my client (likely 15 mailboxes), I purchased 4 and do the setup, but when I going to purchase the 5th one, the system stopped to do that and told me to contact payments@zohocorp.com.

I have contacted
payments@zohocorp.com but no reply, and I'm in a hurry, anyone know how to fix this? 

Thanks!