Something similar happened with my wife once. While she was pregnant with our daughter, the doctor wanted her on blood thinner just to make sure she wouldn't have any clotting issues, which is a problem that runs in her family.
We went to the local CVS for the fifteen day supply, which should have been easy enough. The clerk insisted that it was for only one shot a month and the amount for that one shot was $600, covered by insurance. We argued with her unsuccessfully and wound up having to accept the one shot, every where else was closed. We were on the phone with our insurance by the time we got to the car and CVS pharmacy by the time we got home.
CVS corrected the problem with much bowing and scraping the next morning.