I ran it again without any problem. I may have selected the "ID" in the cart fields for the InvoiceID instead of the "Session ID" the first time. Since there are multiple ID's for cart items and one InvoiceID, that may have been the cause. I'm not sure.
I tried to do the same thing, so that is the only possible difference I can see.
Also, I said TRUE to sandbox and it still came out FALSE in the code. Is there a more detailed tutorial? While using the Wizard to insert the forms, I am not sure at times if some of the fields should be values returned from PayPal that are not available as bindings, or something else. I'm having a slightly hard time matching the Integration Guide to the Wizard.