Princess offers interline discounts on select sailings to Vacations To Go customers who are currently employed with a domestic or international commercial passenger airline (including air traffic controllers), Federal Express, DHL, or United Parcel Service (UPS). In addition, the North American Interline program includes employees of Galileo/Apollo, Worldspan, Amadeus, and Sabre. The spouse, parents and parents-in-law of an interline eligible guest may book one stateroom if the interline eligible guest is not traveling. Retired airline employees are also eligible provided the airline is still in operation (e.g. retired employees of Pan American Airways do not qualify). Qualifying guests must submit a letter of employment verification, or a photocopy of their company identification card, to Vacations To Go at the time of booking.