A bit naughty not to be transparent from the off set, however, they have told you now, which IMHO is better than after you have hit the tyre wall.
£20.00 is a very reasonable fee to reduce the excess from £1500.00 to nil.
If you still feel aggrieved take it out on the GTR's tyres and brakes.
Go have your fun and stop whining.
Alex.