The term 'MAIL IN A BOX' can be solved by the crossword solution 'SHIP', which is a four letter word. Shipping is the process of transporting goods from one location to another, usually by air, land, or sea. It is a common practice for businesses to SHIP products to customers, and for individuals to send packages to friends and family. Mail in a box is a type of shipping where an item is placed in a box and sent through the mail.