No, you're correct that, typically, you can't use a coupon to lower your cost when you're using Medicare and a supplemental insurance policy unless you choose not to use your insurance.
You can use the coupon if you tell the pharmacy not to run the medication through your insurance.
If I have coupons, I compare the cost of my insurance co-pay against the cost of the medication using the coupon then use whichever is less expensive. Usually, my insurance co-pay is less expensive, but it might not be that way for everyone.
Be sure to shop different pharmacies, because prices vary so you might be able to save money just by using a different pharmacy.
Gosh, I don't know the specifics of this. Sometimes doctors really can guide you. Have you called your doctors office or even a pharmacist may be able to guide you.