To be sure you have a seat assignment, arrive at the airport at least 1 hour prior to your flight's departure and check in promptly. Include your name, itinerary number, and date of travel. Or ask for a printed receipt when you check in for your flight. If you are travelling internationally, a paper receipt may be required if you need to change your travel plans en route or if one of your flights is delayed. 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.