Grrr! This should not be so difficult! Nor should CVS be allowed to be so stubborn and still participate in the Medicare program. Once again, as patients we need to be our own advocates. I have worked this out over the years with 4 Walgreens stores in 3 states.
Here is the info from Medicare:
https://www.cms.gov/medicare-coverage-database/view/article.aspx?articleId=52466
Medicare Part B covers nebulizer as durable medical equipment, and solutions as supplies. The doctor must provide the proper diagnostic codes. The pharmacy must enter them, and state whether it is the initial nebulizer Rx because they get a dispensing fee, or a subsequent order.
ALL licensed pharmacies that dispense nebulizers and accept Medicare are able to do this, they may need to be TAUGHT how.

CVS Mail Order and Walgreens have both filled my 3% and 7% orders, though sometimes Walgreens must order in the 7% from their warehouse or supplier.

I have found the best way to handle this is with this document in hand, first in the doctor's office, then at the pharmacy. I wouldn't bother if it was a one time thing, but having the Rx covered saves me $30-50 a month, which is not small change.
Sue