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extra vs

Extra is usually used as an adjective, or a noun:-

Used to describe something added to an existing or usual amount or number.
An item in addition to what is usual or strictly necessary.

For example: We set an extra place in case she brought her new boyfriend.

Surplus can be a noun or an adjective:-

An amount of something left over when requirements have been met; an excess of production or supply over demand.
More than needed or used.

For example: Luckily they were able to return the surplus.