Chances of being caught are probably small, but the money should go back to the insurance company. If you intend to spend the money on new glasses soon then it’s not likely a big issue, but if you want to keep the money for other reasons then that would be benefits fraud.
You can look up many similar cases of benefits fraud where people have worked with businesses and received invoices for items they didn’t purchase or were inflated.
This example isn’t quite the same, but it’s the same idea. The insurance company reimbursed you for an expense that was cancelled.