Prosites checkout page requires login

Hi, would someone explain this to me. Visitors create a blog and activate it. They get a final email telling them their site is ready and get username and password with a login link, e.g. they enter their login details and then the checkout page is displayed ( it asks them to login to upgrade. if they login again then all is good.

Why is it doing this and how do I remove it?

using pay to blog with no free trial

Thanks in advance.
Graham J OConnor