[Rick Seaney is the CEO and cofounder of FareCompare, and columnist for Investopedia. The views expressed by columnists are those of the author and do not necessarily reflect the views of Investopedia.]

We’ve all seen it: a fantastic, almost impossibly cheap deal. Could it be for real? Sometimes, sure. Ultra-discounter Spirit used to be famous for its occasional $9 sale fares (it still has a $9 Fare Club). And just a couple of weeks ago, Wow Air was offering deals from Baltimore to London or Paris for $69.

So when is a deal not a deal? There are four things to watch out for. Finding one of them doesn’t necessarily mean the deal is no good at all, only that it might not be as good as you thought.

1. One-Way Deals

All the deals mentioned above are for travel in one direction, which is how a lot of airlines advertise prices – as one-way fares. The question left unanswered: Will you pay the same dirt-cheap price in both directions? Sometimes it works out that way, often it doesn’t.

Here’s an example for cheapest nonstop flights on a major U.S. airline for travel in October (found on the carrier’s site earlier this week):

  • New York to Berlin, $180
  • Berlin to New York, $491

The total price of the ticket is $671 – still a good deal for Europe, but not quite as good as $360 ($180 + $180).

2. Odd Times to Travel

I recently saw some good fares on a connecting flight between Los Angeles and LaGuardia on a discount carrier with a stop in Denver for $256 roundtrip. Then I took a closer look at the itinerary and saw a big problem; the flight out of New York arrived in Denver about 1:00 a.m. and didn’t take off for LA until 4 p.m. For most of us, that’s too long a layover.

3. Odd Days to Travel

You’ve probably heard this: The cheapest days to fly are usually Tuesday, Wednesday and Saturday – and surprise, surprise, that’s when most sale fares are available. Which is fine if you are free or flexible enough to travel whenever you want. When you aren’t, prices can rise dramatically.

One domestic carrier recently offered flights from $47 each way for fall, and these prices fell mostly on Tuesdays, Wednesdays and Saturdays. See the difference a couple of days makes for roundtrip flights in October from Burbank to San Jose, Calif.

  • Tuesday to Tuesday: $94
  • Sunday to Sunday: $117

Another airline offered these fares for Boston-Baltimore:

  • Tuesday to Tuesday: $98
  • Sunday to Sunday: $192

Want to an even bigger difference? Travel as a family of four; it’s so much easier to save (or spend) a lot more.

4. Fee Add-ons

The biggest fee add-ons are typically found on the cheapest airlines, and that’s true in the U.S. and around the world. Airlines such as Spirit, Wow, Norwegian, Ryanair and so many more offer highly discounted fares and fees galore. Some examples:

Diet Coke – Domestic Flight

  • Spirit – $3
  • American – free

Carry-on bags (prices vary depending on when fee is paid)

  • Frontier Airlines domestic flights – $30 to $60
  • Wow Air transatlantic flights – $39.99 to $99.99
  • United Airlines domestic and international – free

Don’t Forget to Use Your Calculator

When you shop for a flight, the first thing you’ll want to do is compare airfares. Then, before you book – particularly if it’s a stunningly low fare – add in any fees you will need. This might include seat selection, meals and checked bags (see Which U.S.Airline Costs the Least? It Depends). Only then will you know if the the deal is still a deal and worthy of your credit card.

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