This is the only correct answer there can be to this thread.
Call them tomorrow. Do not wait for them to figure it out. There is no possible upside to waiting, only a slightly increased risk of a bad outcome. Tell them what's going on, and be clear that you have no intentions of trying to keep it and are happy to return any undue monies.
In your email, ask that they acknowledge receiving the communication in writing. Until you have their response, you need to keep reminding them, every day.
You need to do email because you need written proof that you noticed it, and did the right thing.
And then call, because it'll be easier to coordinate how you can return the money by phone, and it's hard for them to ignore you on the phone (vs. email). Ask again that they reply to your email ASAP while you're on the phone.
You need to be very, very clear that you only just noticed this, and you're contacting them immediately to rectify the situation, and you need confirmation in writing that (a) you did that, and (b) they know about it.
That way, you've got evidence that you never attempted to steal it. Which, it goes without saying, is really important.