Bug? Correct amount not being passed to PayPal

I've just upgraded to latest version 2.1 on Joomla 1.7.3.

I'm using a simple form with a base amount, and one custom field adding shipping.

The final screen shows the correct amount (ie $63.99 - $60.00 + $3.99 shipping), but when you get to PayPal only the custom field amount is passed (the $3.99)

Removing the custom field resulted in a zero transaction.

I've made it work around for now by putting the full amount in the custom field.