Returning the extra change the cashier gave you by mistake is a sermon nobody hears but God.

Integrity is tested in situations with no witnesses and no consequences, which is most situations. Returning money nobody will miss produces nothing visible: no credit, no story worth telling, only the internal fact of having done it. That is precisely what gives it weight, since it cannot be motivated by reputation.

When the next small chance appears, and it will this month, take it without mentioning it afterward. Notice the pull to tell someone, and let it go. A character built in unwitnessed moments is the only kind that holds up when something larger is at stake.