Baggage help needed?????

i will be travelling with thomas cook in two weeks and will be taking a suitcase i have took on other flights with ryan-air and bmi baby as hand baggage.but when i booked online i did not realise that the suitcase i am using is 4 inches bigger than what thomas cook allow.is it best to add extra luggage online before i go or is it best to go to the airport and pay whatever then?thanks