This type of situations is all about *initiative*.  If you contact them first, it looks like an honest error you caught.  If they discover it and contact you first, then it looks like you tried to get away with something and they caught you.  

Of course, in the latter case, you will protest that it was an honest mistake, but *that just sounds like you're saying that because you don't have any other options now that you got caught*. So you will have zero credibility.   

So Right Now do whatever it takes to get caught on tape trying to tell them ASAP.   If they are closed and don't have voice mail, I would write a note, put it in an envelope, and slide it under their door, making sure to get caught on CCTV doing so.  That will put a timestamp on your efforts.

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My problem with your statement is "getting a loan from a company" is not something one does routinely.  If you had such a pressing life situation that you needed to do so, I would think you'd have waited with bated breath for that check, deposited it the moment it comes in, and *remembered doing so* because it solved the situation.  So finding the check barely 2 months later, it seems super weird to remember what the check was,  but assume the most likely scenario is "I must've forgotten to cash it".  That strains belief. 

Then, since you used mobile/e-deposit into a bank account the first time, *why didn't you do that the second time?* Why **change horses** and go to a check-cashing place (and pay the exorbitant fees there)? Well to get *cash* of course!   Protest as you may, Occam's Razor points to a 19-year-old's naïve hope of ripping off the check cashing shop, which you now got cold feet on, possibly because you did more research on how these things actually work.  

I am not accusing you of that; I am saying that's what people will assume, and it's *on you* to show, by fact and act, that this is not true.

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The other possibility is that you do get such loans routinely enough to really get confused.  This would indicate a different problem, along with dealing with expensive check-cashing shops in the first place.  That is a problem worth asking about, because most people do not routinely get loans and never set foot in check cashing shops. 

Lastly it might be worth mentioning those online scams where "employers" "pay" you to cash checks into your personal bank accounts and send money on to them.  At best they are laundering money; at worst they sent you a bad check that will ultimately bounce after a very long delay.