Several Democratic lawmakers want airlines to disclose the cost of handling a bag or changing a flight.
Airlines globally brought in an estimated $57 billion in ancillary revenue last year, according to IdeaWorks Company.
Lawmakers are also want the DOT to ensure airlines have policies to make sure families sitting together.
Several lawmakers are questioning airlines and the Department of Transportation about the fees carriers charge passengers to check their luggage or select a seat.
The queries came after two U.S. airlines — JetBlue Airways JBLU and United Airlines UAL — raised the price to check a first bag on their flights by $5 to $30. JetBlue also increased cancellation and date-change fees, following a similar move by Alaska Airlines ALK earlier this year. United recently said by the end of the year it would charge travelers to select preferred seats, those with standard legroom located toward the front of the cabin.
In a letter to 11 airlines dated Sept. 6, Sens. Ed Markey (D-Mass.) and Richard Blumenthal (D-Conn.), and Rep. Steve Cohen (D-Tenn.), said passengers drawn to low airfares are then "bombarded with a rash of airline fees that can drastically increase the overall cost of flying." They asked the carriers to disclose the cost of processing a checked bag and whether the cost to change a passengers' ticket has increased over the past decade.
Ancillary fees, which are not subject to the same 7.5 percent tax as airfare, provide an additional revenue stream to airlines, particularly as oil prices have climbed. U.S. airlines collected $4.6 billion in checked luggage fees last year, up by nearly a third since 2012, according to the Department of Transportation.
"To the extent that fees or other surcharges help airlines generate more revenues or avoid costs for services that some passengers may not value, they help overcome an economic climate in which costs are rising steadily, as they are today," said Alison McAfee, a spokeswoman for Airlines for America, an industry group that represents American Airlines AAL , JetBlue, United, Southwest LUV and others.
U.S. airlines began charging passengers to check their luggage a decade ago as fuel prices spiked. Since then new fees have increased along with no-frills tickets that in exchange for a low fare does not offer upgrades, changes or a seat assignment ahead of time, which could mean that passengers traveling together could end up being split up.
In a separate letter to the Department of Transportation dated Sept. 10, Sen. Ed Markey, (D-Mass.) and nine other Democratic senators asked the agency that oversees airlines why it hasn't instructed airlines to come up with a policy to ensure air families with children can sit together.
The DOT reviewed airlines' family seating policies and posted "practical tips" on its website for families that want to sit together, but it does not require airlines accommodate these families.
Airlines that offer basic economy fares that do not include seat assignments remind travelers several times during booking about the restrictions of the ticket and say gate agents and other customer service representatives work with travelers flying with children so they are seated with at least one parent or other adult in his or her party.
"Delta works with customers on a case-by-case basis to ensure their travel needs are met when they fly with us," said Delta Air Lines spokeswoman Kate Modolo. "We encourage customers to call Delta reservations to talk with one of our representatives if they have questions about their child's seating or any other concerns prior to flying."
More From CNBC