What is the expiation for taking extra bags from a store, to use them as a trash can, knowing that these bags are free for customers?

If you took bags from a store without permission, you must return what you took if it is still remaining, or something similar if available, otherwise its value, unless an authorized person in the store absolves you. This is based on the Prophet's (peace and blessings be upon him) saying: "The hand is responsible for what it has taken until it returns it," and "The wealth of a person is not lawful without his willing consent."