Hidden fees have become a big part of air travel. In fact, they can sometimes add up to more than the cost of the tickets themselves.

So, how does one know if they are getting their ticket for the best price? An airline passenger advocate says that it can be hard to tell.

“They go on these websites and they look to see what the lowest fare is,” says Flyers Rights founder Paul Hudson. “However, what they don’t see on all these websites is all the extra fees involved.”

For example, the days of getting two free checked bags are now gone. All major airlines but Southwest charge from the first checked bag. It can be from $30 to $65 for each bag. If a bag is overweight, the cost can be anywhere from $50-$200 more. And each airline can set its own weight limits.

Three discount airlines – Frontier, Spirit and Allegiant – now even charge for carry-on bags.

When paying for carry-ons, when you pay is critical. For example, for Spirit Airlines, paying for carry-ons when you book your ticket is $35. If you pay at online check-in, the cost is $45. If you pay at the airport, it is $55 and if you pay at the gate, it is $65 per carry-on.

“It goes on and on. So, what was a $25 ticket can end up being a $300 fee,” says Hudson.

What should travelers do? Hudson says that instead of just looking at the ticket price, take fees into consideration. Travelers should check their credit cards. Some will offer free checked luggage or waivers on other fees.

And if you do have to pay, check the rules. Paying early is often cheaper than paying the fees later.