Sometimes insurance can be hard to figure out. There is a deductible, an amount that you have to pay before insurance starts to kick in. There is sometimes a separate deductible for medical and pharmacy benefits. Once you reach your deductible then you should be covered with a co-pay. If that $400 charge is both your deductible and your co-pay you may find the next order much more affordable.
ps. It is for this reason that I often stock up on all my supplies before the end of the year.