Extra and Overage

Extra noun - An interchangeable part or piece of equipment that is kept on hand for replacement of an original.

Overage and extra are semantically related in excess topic. In some cases you can use "Overage" instead a noun "Extra", when it comes to topics like surplus.

Overage noun - The state or an instance of going beyond what is usual, proper, or needed.

Extra and overage are semantically related in excess topic. Sometimes you can use "Extra" instead a noun "Overage", if it concerns topics such as surplus.