A carton with an elongated expandable flap intended to hold items in a vehicle. Formed of a unitary sheet of blank material. The carton comprises a plurality of panels that Includes two bottom panels, a front wall, a rear wall, first side wall, and a second side wall that define the cavity. Two generally rectangular, opposed first and second side wall are connected to the remaining opposed edges of the bottom panels and project upward therefrom. Each front and rear wall is connected at one of its edges to one of two opposed edges of the bottom panels and projects upward therefrom. A different embodiment could comprise an integrated top panel for closure.

A locking receiving portion is defined by a first inclined line extending toward a third panel side on a first panel from a branching point on a fold line between the first panel and a second panel, a second inclined line extending toward the third panel side on the second panel from the branching point, and a cut portion extending between a distal end of the first inclined line and a distal end of the second inclined line. The locking receiving portion extends in an inclined manner between the first panel and the second panel in an assembled state. A locking portion is formed on the third panel, and configured to be locked to a cut portion side of the locking receiving portion in an assembled state.