I can't see why you'd be charged more. Prices on the website are for 'first additional bag' and 'other additional bag' rather than third or fourth bag.

So you've paid (correctly IMHO) for your first additional bag beyond your silver limit of two bags.

EDIT: I've just seen your comment about £34 compared to the usual £112 charge - so you think you somehow fluked a good deal, is that it?

Where are you travelling?