There could be a costly unintended consequence for airlines that charge checked baggage fees: a US federal court ruled that a passenger who was charged $15 to check a bag has the right to sue for a refund because the baggage was delivered late.

The San Francisco Chronicle reported that the big threat to airlines would not come from individual travellers, but rather if cases like the one ruled on by the U.S. 9th Circuit Court of Appeals are bundled into class-action lawsuits.

Airlines and the trade group Air Transport Association of America opposed the suit filed by  Hayley Hickcox-Huffman, a California resident who paid $15 in 2009 to check a bag on a U.S. Airways flight between Colorado Springs, Colorado, and San Luis Obispo, California. The bag was delivered a day late.

The court said that US Airways’ stated “committed to … provide on-time baggage delivery” could be the basis of a lawsuit by Hickcox-Huffman since it was a voluntary promise made by the airline to its customers, and not based on airline regulations.

Federal deregulation laws generally bar lawsuits against airlines on the basis of pricing, service, and routes, but the appeals court voted 3-0 in favor of a ruling stating that the airline extended to Hickcox-Huffman “a contract to deliver the bag when she landed.”

On a more positive note, a recent report found that airlines’ lost-baggage rates are at an all-time low. As for airlines, they’re earning more than ever on checked-baggage fees.