Here's the thing. The best thing to do is call the blackmailer's bluff. Ignore him. Don't pay this extortionist. You know what you did, you know of the possible consequences of your actions. You did this with your eyes open. Now is not the time to grow a conscience. Your only choices are as follows:
1. Pay the amount asked and hope the blackmailer goes away. Note: He won't. You just gave him ammo to continue blackmailing you for years to come.
2. Don't pay and see if he actually has enough information on you to blow your academic dishonesty sky-high. Which I doubt. Then again, I am not aware of the kind of personal data he required when you placed your order that might allow him to end your academic career and professional future in one blow.
Usually, these people are just out there looking for gullible people to victimize. I highly suggest going for option 2. There's a high chance he isn't capable of making good on this threat and your academic dishonesty secret will remain just that, a secret. If you pay, then make sure you have the kind of money in your account that you can just throw away each and every time he harasses and threatens you. Don't be a fool twice over.