think I can help

First of all, do you work or do you have a working spouse and that is why your commerical is primary? If you are not working and you do not have a working spouse and you are 65 and over then medicare would be your primary. There are a lot of regulations re this. Medicare DOES act as a secondary but it is really complicated. IF we get the above straightened out as to who is really primary then we can deal with the next thing and that is if you commercial IS primary, then the outpatient clinic is mandated to take your commericals payment in full and they are not allowed to bill you!!!!!
You cannot chose who is going to be primary and who is secondary, unfortunately. I agree that things would be better if medicare was primary because your bills would be smaller to begin with.
Secondly, we need to talk about apples and apples. Medicare D is a program just for medications and really is not pertinent here because you said that you received txs at a clinic and not at home. I believe your ivig then falls under your medical plan and has nothing to do with medicare D. You do not by any small chance have a medicare HMO do you? These are tricky and rare. I assumer you have traditional medicare A and B. I sure do not know a whole lot about cidp but I do know about this stuff!!! Ha!