If you plan to fly Spirit or Frontier this holiday season, beware. The two budget airlines increased their checked bag fees during this peak travel time.

For Spirit, there’s a temporary $2 checked bag fee Dec. 18 to Jan. 4. The airline instituted this higher fee last year, and again over the summer. Frontier is a little more convoluted, using a “dynamic” pricing structure that keeps the fee a secret until you input the origin and destination of your flight, as well as dates of travel on their website. The airline is implementing an additional $2 seasonal fee in some cases.

Paul Berry, spokeperson, Spirit, attributes this to the “added costs for baggage handling, manpower, airport infrastructure and added weight to aircraft, which burns more jet fuel. Most airlines respond to these added costs by jacking up their ticket prices for everyone. However, we temporarily charge only those who are checking bags an extra $2 to cover the added cost.”

The U.S. Department of Transportation reported the 27 major U.S. airlines netted more than $3.5 billion in baggage fees last year.