You should have mentioned in your first E-mail that you bought it more than 90 days ago. If they then agree to exchange it and later on, after having received the game, say they can't help you, you point to the E-mail where the employee did say it was allowed.
Now you have a chance that they'll reject it.
Plus, serial numbers do not go invalid over night. You do not even need to connect to the Internet to activate them, so there is a 99.9% probability that you did not type down the numbers correctly.
The remaining 00,1% being a Gremlin.