@northland Hmm, that's a question for a Medicare specialist at your insurance company.

Here is what my very experienced local pharmacist explained to me - the pharmacy inputs the prescription info into your account at the pharmacy, there is a built-in table showing your insurance of record, the formulary (approved drugs) under your plan, the full cost and your co-pay. It may also contain diagnostic codes for drugs you take routinely.

The billing issue has happened to me both ways - sometimes I get a bill (from the pharmacy, not Medicare) for an additional cost. Sometimes I get a check from the pharmacy because Medicare reimbursed or allowed more than expected.
If you are referring to the "Statement of Benefits" from Medicare, it is NOT a bill. It is a statement of what the drugstore charged, what Medicare "allows" and the difference. You may or may not be billed for the difference - it depends on your supplemental (Part D in this case) insurance policy terms. For example, my supplement says all Tier 1 blood pressure and asthma meds and some diabetes meds cost no more than $5.00 for a 3 month supply. The insurer picks up the difference, no matter what the pharmacy charges

Another, highly confusing, exception - drugs inhaled through a nebulizer (albuterol, steroids, saline solution) - are not covered under Part D as a drug. They are covered under Part B as a "supply" to be used in a piece of "durable medical equipment." Some pharmacies erroneously submit them under Part D and Medicare "rejects" them - making your cost higher. When this happens, you need to go back to the pharmacy and tell them to resubmit under Part B using the correct codes. This especially happens the first time at a new pharmacy, or when a new pharmacy tech does the input.

What exactly does your statement of benefits have to say?
Sue