Já viajei com bagagem fora de formato por várias companhias... Por exemplo na TAP nunca consegui-a acrescentar na reserva, tinha sempre de contactar o apoio cliente para adicionar e pagava na hora do voo...
À partida não terás problemas desde que a bagagem esteja dentro das dimensões permitidas como bagagem fora de formato...
Quanto a mim, o melhor será contactares directamente a companhia e questionar acerca do assunto...
"We accept individual items up to: length 250cm, height 79cm, width 112cm. The total circumference (L+H+W) must not exceed 300 cm. The size limit does not apply to a double bass."
"Unless otherwise specified, oversized baggage can be ordered at the time of booking online and via our Contact Centre. If you want to add oversized baggage after you have made a reservation, log in to “My Reservations” and follow the prompts or phone our Contact Centre.
The applicable charges must be paid at the time of purchase. All passengers must bring their travel receipt showing proof of the baggage purchased to the airport on the day of travel. Higher rates will apply to oversized baggage ordered at the airport. All baggage charges are non-refundable."