I agree a well done should be given face to face to show you appreciate the service you received.
I personally don't treat a airline employee any different from hotel,train, bus,ferry, shop,call centre or any other service industry staff to be honest. I don't go writing letters because the girl at the corner shop or the ticket inspector on the train were doing their jobs the way they should be carried out. That would end up being a full time job.
If they go way beyond the call of duty then yes a letter of praise should be sent but on this occasion it does seem that they were only carrying out their duties the way they were trained to do.

Is a genuine smile and a thank you not good enough?