Why airlines are raising baggage fees — and charging you more at the airport

Airlines are raising their prices to check a bag — again. Just how much you it will cost you, however, depends on when you pay for the service.

United Airlines, American Airlines and JetBlue Airways are among the carriers that have raised the price to check bags this year. Each of them charge customers more if they check their bags at the airport or close to their departure compared with paying to check a bag online in advance.

Carriers are encouraging customers to pay to check their bags ahead of their flight, an approach the airlines argue will free up employees at check-in areas and get travelers to their gates faster.

Earlier this week, American Airlines raised its checked bag fees for the first time in more than five years and adopted the two-tiered strategy that United, JetBlue and several budget airlines already have.

American Airlines customers traveling in coach will pay $35 to check a first bag for domestic flights if the service is booked online in advance, or $40 if they purchase the option at the airport, the carrier said Tuesday. American Airlines previously charged $30 for either service.

There are exemptions. Customers who have certain airline or other rewards credit cards, are traveling in a top-tier class or have elite frequent flyer status generally can check at least one bag for free on domestic or short international flights.

"It allows our team members to spend more time with customers who require additional assistance with their travel journey," an American Airlines spokeswoman told CNBC.

American this week also said it is reducing fees for slightly overweight bags, which used to force some travelers to remove items from their bags last-minute at the airport to meet the threshold.

The different fee tiers