Do you fly with United Airlines often? If you do, you might be helping the airline rake in billions in fees annually. Your last-minute flight changes and heavy luggage could turn your inexpensive ticket into a pricey purchase in the blink of an eye. United Airlines is the industry leader when it comes to making revenue unrelated to ticket sales.

According to a study by the IdeaWorksCompany, United Airlines made $6.2 billion in non-ticket revenue in 2015 alone. That's $2.5 billion more than Delta and $4 billion more than Southwest, and it's about triple of what United made back in 2008.

A lot of that revenue comes from fees, but about half of the $6 billion haul came from United's partnership with Chase. United sold nearly $3 billion worth of miles to Chase in 2015, which distributes them to holders of United's co-branded line of credit cards.

Frequent flyers can balance out United's fees with the rewards of co-branded credit cards such as United℠ Explorer Card, but you should still try to avoid paying more than you need to. Here are some of the hidden fees and pricey extras you need to be aware of before booking your flight with United.

Fees To Watch Out For

There are several fees that can easily sneak up on you when you fly with United Airlines. The big one to watch out for is the airline's notoriously high baggage fee.

All passengers are entitled to 1 free carry-on bag. A single checked bag will cost you $25. Members of MileagePlus, United's frequent flyer program, get 1 free carry-on and 1 free checked bag, but a fee of $35 is tacked on for a second checked bag.

The fees don't stop there. Additional bags beyond that will cost you between $100 and $200. There is also the potential to be charged a fee if your bag is over the airline's standard weight limit. Overweight bags are charged with a fee between $100 and $200. The price can shoot up to as much as $400 for certain international flights.

Now, you can see how the luggage fees you incur during your trip can easily add up to be as much as the cost of your ticket if you're not careful. Another fee that most flyers are unaware of is the $25 charge for booking a flight over the phone.

The Fees Hidden In Discounted Tickets

The irony of the situation is that the people scooping up the cheapest tickets are often the ones paying the heftiest fees. United Airlines attaches heavy cancellation penalties to discounted tickets.

Most tickets sold at the lowest price tier are unforgiving when it comes to missed flights or itinerary changes. Regular priced tickets allow passengers to adjust travel plans for much smaller fees.

However, economy-level tickets don't always come with this same perk. This means that you could be on the hook for the total cost of your ticket if you miss a flight or need to cancel a trip due to unforeseen circumstances.

It's important to make sure you aren't putting yourself in a position to lose money on tickets just because you're focused on booking the lowest priced flight possible. But then again, most passengers are happy to go along with the risk of higher cancellation fees as long as their own plans don't change.

One way to avoid high fees is to join United's MileagePlus frequent flyer program and sign up for a co-branded credit card from Chase. MileagePlus members and cardholders get lower fees and more flexibility than non-member passengers, plus plenty of other perks.