something else that may be usefull information, when in paypal the error message is :
Please enter an amount greater than zero.
Your shopping cart is empty.
if I then press 'Update Cart' the screen changes from my custom profile to one for 'Great Intentions, Inc' :s
HAving no joy with the sandbox, or live paypal.
Also if I manually test IPN I get an error message complaining about an invalid Transaction ID (Set to PAYPAL00000000000)