"Box" in English can be translated into Spanish as caja, cajón, or estuche. What exactly is the difference between these three words? What types of boxes are translated as each? Which would best describe a shipping box used for sending something in the mail?
The most common one, at least in Spain, is "caja".
"Cajón", though it is defined as "caja" by the Real Academia Dictionary, refers usually to big, sturdy boxes. Like made of wood, instead of cardboard. But in Spain, "cajón" is usually used in its "drawer" meaning.
And "estuche" generally refers to small boxes, for storing pens, jewels, etc. In some cases it used for bigger things. For example, music instruments, like guitars, are kept in "estuches". But those are quite specific cases.
So, summarizing, and generalizing:
For the shipping box, I would say "caja". And, when in doubt, use "caja grande", "caja pequeña", "cajita" ...