I am new to Zoho Books and am just starting to experiment. I sent myself an invoice at a personal test email address, which I paid. 

When I go back to the email I received and click "View Invoice", I get taken to a zohosecurepay page 

which shows the paid invoice (good) and a sidebar including the link "One powerful portal to manage all your transactions with us." When I click the Sign Up link, I'm taken to a books.zohosecure.com "Sign Up to the Portal" page.

There's a dropdown containing my partially obscured test email address; when I select it and say "sign up" I get the error "The Customer Portal has already been enabled for this email address. Try signing in to the portal instead, or sign up using a different email address."

Problem 1: There's no way to enter a different email address; I can only choose from the drop down list.

Problem 2: If I go to the Sign In page and enter the same email address, I get the error "This account does not exist."

Problem 3: On the same zohosecurepay  invoice page, there's a link for "retainer invoices." Clicking on that gets me "Resource not found [404]" and no way to get back to the previous page or anywhere else.