The cashier's check they send you will be fake. You deposit it, and then they'll say "oops, we paid you too much" or "can you send money to the shipping company" and ask you to take most of the money out in cash and send a Western Union, buy a prepaid card at Walmart, etc., and then send the money to a third party.
The bank will make the funds available to you by the next day, per Regulation CC, since a cashier's check is considered a "next-day item." Usually they will make the funds available immediately, as a courtesy. However, it takes a couple of days for the check info to get transmitted from your bank to the Federal Reserve (or possibly a local/regional clearinghouse), downloaded by the paying bank (the one whose routing number is on the cashier's check), and discovered not to match their cashier's check records. At that point they'll initiate a chargeback to your bank, essentially saying "hey, this check was no good," and your bank will have to take the funds back out of your account. Problem is, if you've spent the money, that might result in your account being severely overdrawn. Since the money you sent was via an instant method like prepaid card or Western Union, it's essentially untraceable and unrecoverable, so you're SOL and you have to pay your bank back for the amount of your overdrawn account.
Source: I am a fraud investigator at a bank. I see variations on this scam dozens of times a week.