I avoid these extra charges like the plague - never pay by credit card, always hand baggage only (it's easily done for 2-5 days) and I only paid for seats once. I took the long suffering for a romantic weekend in Venice (on Jet2 actually), booked extra legroom seats 1A & 1B, and they were nearest to the door - let me tell you, on a fresh Edinburgh morning, we were frozen solid! And I paid extra for that!