As any traveler who has ever flown with a low-cost carrier like Spirit or Allegiant knows, sometimes the cheapest flight is not actually the cheapest. Spirit earns an average of $51 from each passenger in extra fees, and Allegiant is not far behind at $45. So, that great deal you found for your upcoming flight to Los Angeles? It may not be as much of a bargain as it seems. But there are simple steps you can take to avoid those budget-busting costs. Here are five do's and don'ts to make sure your next low-cost ticket is truly as affordable as advertised.See: 6 Travel Fees You Can Easily AvoidTo dodge fees on low-cost carriers, you have to think about luggage a little differently. For instance, Spirit actually charges more for a carry-on bag ($35) than a checked bag ($30), if you pay for the bag at booking. All low-cost carriers, however, allow travelers to board with one personal item (a purse, backpack or laptop bag) free of charge. If you want to avoid the high fees, plan to take one personal item on board and share a checked bag with your travel companion. And remember to pay the baggage fees at the time of booking. The longer you wait, the more expensive it gets.If you can, try to print your boarding pass at home or at the office. If you don't have access to a printer, it'll only cost $2 to print your Spirit ticket at the airport kiosk. But don't ask an airline agent to do it for you, as that will set you back $10 per boarding pass on Spirit and $5 per ticket on Allegiant.This is true for both low-cost and legacy carriers: don't call them to book your flight. If you do call for booking, be prepared to shell out $10 to $25 per ticket if you book over the phone with the airline's customer service representative. If you want to avoid that fee, use the airline's website or an app like Hopper. Note that some airlines, such as Spirit and Allegiant, do include a minor online booking fee, but it's always less expensive than the phone booking fee. And remember: You can avoid booking fees altogether by purchasing your tickets at the airport.See: 11 Easy Ways to Slash Travel CostsIf you need to pay for any extra services like a checked bag, make sure you pay for it at the time of booking, when you'll owe the least amount of money. If you neglect to pay for your bags when you initially purchase your ticket, you'll have another chance at online check-in (when the prices are just a bit higher). But do not put off handling these fees until you arrive at the airport. That $30 checked bag on Spirit? It's $50 if you pay for it at the reservation desk and $100 if you wait until you're at the gate.Low-cost carriers charge additional fees for the luxury of selecting your seat before check-in. So if you want to make sure you're sitting with your travel companions, you'll have to pay for it. But if you don't mind sitting by strangers, you can save a few dollars and leave it to chance that you'll still be able to sit together when seats are assigned at online check-in.See: Luxe for Less: How to Visit 5 Big Cities on a BudgetAbout the author: Dakota Smith is an editor at Hopper, the travel app that tells you when to fly and buy. He writes articles on air travel and offers expert advice backed by Hopper's unique data. Follow Hopper for more tips and deals on Facebook, Twitter and Pinterest.