If you're checking car-rental prices via online travel agency Orbitz, be warned that National, Alamo and Enterprise will be missing from the comparison. Enterprise Holdings, which owns and operates Alamo Rent A Car, National Car Rental and Enterprise Rent-A-Car brands, has ended its relationship with Orbitz.com and its sister site CheapTickets.com. The company took the decision after Orbitz de-listed National and Alamo. The two companies had been in negotiations about commission rates. Late last year, American Airlines pulled out of Orbitz after failed contract negotiations. Travelers can still find the companies quotes on Priceline and Travelocity, and can book directly at alamo.com, enterprise.com and nationalcar.com.