Do I need to return an extra item when a company makes a mistake in my favor?

[Jan]

If you're a completely honest person, you'll ship it back.
However, online companies cater for delays and mistakes and that's incorporated in the price you pay for the product.
Unless your contract specifically says you HAVE TO return it, you don't legally need to.

Your mother has a lot of empathy, she can put herself in the supplier's shoes. If you would be the supplier, would you want to have it returned?
Whether you do or don't, you won't go to jail for it or pay a fine but it's you who has to live with your actions.