Is this Apple USB-C Power Adapter original?

To guarantee that it is an original Apple power adapter you should purchase one from the Apple store.

Buy 20W USB-C Power Adapter - Apple

If someone is offering the original Apple 20W power adapter at a drastically reduced price then it may be counterfeit or damaged.

No one can determine it’s authenticity from a box. If a manufacturer is smart enough to build a counterfeit charge cube, they are probably smart enough to build a convincing box.

Why do you believe it is not original?