See elijah's esponse to the question you asked. https://www.southwestaircommunity.com/t5/Family-Travel/Checked-Car-Seat/m-p/93647#M1701 But to answer your question, boxes are acceptable "luggage." You'd pay exra if the box is more than 62 inches (height+length+width) or it it weighs more than 50 pounds
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