Porter

This airline does not sell seats for instruments.

Porter rules

Small instruments such as a violin, guitar, flute or trumpet may travel as your standard carry-on.
The instrument must be in a hard-shell case, weigh no more than 9 kg, and fit under the seat or in the overhead bin.
Seats cannot be bought for instruments.
Staff may require a gate-check if it cannot be stowed safely. Gate-checked instruments are returned at the gate on arrival.
Check your fare: the lowest fares may not include a standard carry-on.

Porter instrument policy

Small instruments in a hard-shell case, up to 9 kg, are permitted as standard carry-on.

https://www.flyporter.com/en-us/travel-information/baggage/special-items