Pack light or pay more.

That’s the message two U.S. airlines have given their customers with announcements they will be charging extra for checking a second piece of luggage.

In early February, United Airlines announced it will start charging $50 round trip for a second piece of checked baggage. U.S. Airways followed suit three weeks later. The new fees don’t go into effect until May 5, but will apply to tickets purchased on or after the date the new policies were announced.

In United’s case, the new checked bag fees will apply to customers who have bought nonrefundable tickets for domestic travel as well as travel to and from Canada. It does not extend to international travel or to United customers enrolled in its top-tier frequent-flyer programs. The U.S. Airways policy is similar to United’s except that it extends to international travel as well. U.S. Airways is also exempting active military personnel and unaccompanied minors from paying the extra bag fee.

In a statement announcing the new fees, United said that its research shows that about 25 percent of its passengers check a second bag. According to United, the fees will generate $100 million for the company in cost savings and new revenue.

With airlines under financial pressure from skyrocketing fuel prices, some airline analysts believe other carriers will follow the example of United and U.S. Airways. Even before those two airlines announced the new fees, Southwest Airlines in January had implemented a $50 fee (round trip) for checking a third bag.

Other airlines still allow two bags but are hiking fees for excess and overweight baggage. Delta Air Lines, for example, has increased its excess baggage fees (one way) from $75 to $80 for three to five excess pieces of luggage and from $50 to $80 for bags weighing from 51 to 70 pounds. Discount carrier Spirit Airlines, which already charged for checked baggage, recently increased fees from $5 to $10 if a ticket is purchased online and $10 to $20 when purchased at the airport.