Hello,

There's no additional charge for Economy seating in the upper deck, they're treated the same as downstairs.

If there are any available when you come to check in you can assign them to yourself free of charge. Though as you are a Flying Club member (presumably Red?) you will be able to assign yourself a seat at no cost within 72 hours of departure, just make sure you have your FC number added to the booking.

Hope that helps. :-)