The thing to remember is even if it is a Medicare replacement plan, it is handled by a commercial carrier. For example, American Progressive Today's Options doesn't require providers to 'enroll' with them. By submitting the claim on behalf of the patient you are considered 'in network'. It is important that you know each plan's rules. They do have to abide by Medicare rules for what they allow etc, but they also instill there own as well on administrative things. There should always be an allowed amount when dealing with Medicare (even replacement plans) so I would be careful when charging the patient. You may want to call on one and get an explanation (specifically what is the patient responsibility).
Just when you get it all straight - it will change! That's why we all love this business!
Michele