If you already have a subscription and wish to purchase more mailboxes, do the following:
1. Sign in to your Namecheap account.
2. Go to Domain List menu on your left and click on caret to expand the associated services list (make sure All Products is selected in the top right corner):
4. Scroll down to the section with your current mailboxes. Click on a Buy Mailbox option from the dropdown menu and you will be redirected to the payment page: