Here are a couple more Tea Commerce "features" which should be added to the documentation:

Auto assigned properties on the Order object

• Order.CustomerId - If a user is signed in (as evidenced by Membership.GetUser()) at the time the GeneratePaymentForm() method is called, then User.ProviderUserKey is assigned to Order.CustomerId.
• Order.PaymentInformation.FirstName, PaymentInformation.LastName, and PaymentInformation.Email - if you add "firstName", "lastName", and "email" properties to the order's custom properties, then these properties will be assigned to the respective PaymentInformation properties whenever the order is saved. These PaymentInformation properties end up being saved as FirstName, LastName, and Email fields in the TeaCommerce_Order table in the database. The typical place to define the custom properties is via the call to AddOrUpdateOrderProperties as documented in Tea Commerce.