If you're traveling on your own, no problem; just book your tickets. All you have to do is compare prices on an airfare search site because no single airline always has the cheapest flights (see Which U.S. Airline Costs the Least? It Depends).

But what if you're traveling with family and friends? Here's where it gets interesting. You could shop the usual way – entering the total number of passengers instead of just one – but doing that might mean you spend more than you have to. Here's a special technique that can ensure that you're charged the lowest possible fare for each ticket you buy.

The Reservation System Quirk

The reason you should never buy more than one ticket in the same reservation, at least to start, is a quirk in airline reservation systems that requires every ticket purchased in a single reservation (or transaction) be the same price. If there aren't enough of the cheapest-priced tickets available, the system moves up to the next highest level of ticket prices where there are enough for the transaction and you will pay the higher price for all of them.

To help make sense of this, it's useful to know that there are about 10 different price points on each plane; there are even multiple price points in economy class and prices vary depending on when you buy your tickets.

Let's say you're seated in a middle seat in coach and paid $100 for your ticket. To your left is a woman who paid the next lowest fare of $125 and to your right is a fellow who paid the top economy price of $140. I'm pulling these figures out of the air for the purpose of this column but all are well within the range of possibility.

Now say you will fly as a family of four. You buy your tickets in a single transaction as usual; if there are four tickets available at the lowest price, terrific. You got the $400 deal.

But let's say there are only two of the cheapest tickets left. If you buy all four tickets in one transaction, you will not pay $100 + $100 and $125 + $125 for a total of $450. Instead, you will pay a total of $500 or $125 for each and every ticket. Remember the system quirk? The system will not split up a group to mix ticket prices because again, all tickets sold in a single transaction must be sold for the same price. When there are not enough of the cheaper-priced tickets to match the number of passengers, the reservations system automatically bumps up to a price tier where there are enough same-priced tickets.

This can be avoided, though. All it takes is a few extra moments of your time.

Purchase Tickets One at a Time

Begin shopping by opening two separate booking transactions. I open up two different browsers, then do the following.

  • Open a booking for the total number of people in your party
  • Open a booking for one person

If the price in the browser for the single ticket is cheaper than the price offered in the total booking browser, you know there is at least one cheaper seat available. Buy it, and repeat the process of buying a cheaper ticket one at a time. You may end up buying all the tickets you need, one at a time.

If the price of one ticket rises to that in your "total booking" browser, you can use that second browser to order the remaining seats you need all at once. Or keep using the same browser and execute a multiple order for the remaining tickets at the higher price.

You may end up using one or both browsers, depending on which ticket prices you found and what's easiest for you.

Final note: In case of error, U.S. airlines are required to give you a 24-hour ‘change-your-mind’ grace period. Most offer a straight refund within this period, but American Airlines’ policy is to allow shoppers to hold a booking for 24 hours (with no price change) without paying for it so keep this in mind.

The Bottom Line

Never automatically buy more than one ticket at a time to avoid the reservations-system quirk that could push ticket prices up to a higher category. Take the extra time to check prices and split up your transactions if needed to save the maximum amount. For more about saving money, see Airline Ticket Prices: What a Difference a Day Makes and An Insider's Guide: Finding Cheap Airline Tickets.

Rick Seaney is the CEO and cofounder of FareCompare.