My Medicare Part D plan, through AARP, has a tiered copay amount for a 30 day supply of medication.
One of the ways to cheat a customer is to charge for two months copays (and those copays keep going up drastically each year) if they estimate that a liquid medication will last anything over 30 days. So for example if a medication is estimated to last 35 days then they will charge you for 60 days. They will not consider prorating the copay amount to cover 35 days. Imagine, someone might accuse them of being fair and not providing enough money for their executive bonuses.
Added: Turns out that two copays is more (by a couple dollars) than I would pay for the same 10ml bottle if I bought the drug from CostCo without insurance.
Insurance you can believe in?