You can not collect money up front for services such as 99051. You can also NOT charge extra for walk-ins over scheduled patients. This is ILLEGAL. You CAN however ask a patient to sign an advance beneficiary notice if you think their insurance will not pay for the services. This would hold them financially liable after the services are denied.
If you do charge extra in advance for the 99051, you would have to refund the money to the patient after insurance pays, as you are not legally able to charge the patient the difference between the allowed amount and the billed amount. So if the insurance pays 100%, there is no balance, and a refund must be issued. If an insurance company pays at 80% but has you adjust the balance to zero, stating patient responsibility zero, you still have to issue a refund. Also, if the insurance company states that the services are not billable (or what have you), and tell you to adjust to zero and issue no payment (and still state patient responsibility zero), you again would have to issue a refund to the patient.
The only kind of "extra" charges you are allowed to charge a patient for is for returned/bounced checks, and if your office has a no-show policy that results in a charge.