Check with the airline about its policies before you depart. Some employers require paper receipts for business travel expense reports. Some companies accept printouts of confirmation e-mails, but check your company's policy to find out what is acceptable documentation. I've seen a better flight price advertised elsewhere. There are several reasons this may occur.
Airlines generally restrict the number of seats that can be sold.
Most discounted fares have restrictions such as a minimum stay requirement or even the time of day you can travel. Occasionally a special advertised fare may only be available for purchase from a particular company. The advertisement should outline those restrictions. Airlines generally require that ticket purchases using either frequent flyer miles or travel vouchers be handled directly at one of their locations. The same is true of vouchers issued by a variety of travel clubs.