Internet shopping is seenasa cheaper alternative to shopping on the high street. In this sentence the underlined word is used as a/an -

As as a preposition:
We use as with a noun to refer to the role or purpose of a person or thing.
Example:
- I worked as a waiter when I was a student. Most of us did.
- Internet shopping is seen as a cheaper alternative to shopping on the high street.

Source: Cambridge Dictionary