Sending custom fields to PayPal

Hello all,

I have a custom field in my eCart checkout labelled 'delivery times' where a viewer can choose an option as to when they would prefer their item to be delivered. Once they choose this option on the checkout page it is stored as a session variable on the 'confirm' page. This seems to work ok as on the confirm page I can see the delivery option chosen, so I know that the session is being stored correctly.

I can't see though how to make this appear in the PayPal system, or in the email that PayPal subsequently send out to the merchant with the order details on. In my form I've given it the id of 'custom' to be sent to PayPal, but it doesn't seem to appear anywhere in the PayPal system.

Anyone got any ideas how I would do this?

Thanks.