Answer:
The number of boxes in an airmail shipment is discrete.
Here is the trick: If you can count it with your fingers, it is discrete, otherwise it is continuous.
*Assume you have a lot of fingers sometimes.
An example of something discrete would be the number of people in a room, because you cannot ever have half a person it means you will always have a whole number and thus discrete.
Something continuous would be how tall you are. You may tell everyone that you are 5'8 but really you are probably 5 foot 8.10444124(some crazy long decimal) inches. Additionalyl people are constantly growing and shrinking at all times throughout their lives due to the gravity, bad posture and natural growth thus it is variable that is varying and continous.