I have set up Fastspring payments and it works fine, but when a user wants to upgrade halfway through a month, the full price is shown, rather than a price which discounts what they have already paid for the month. I know that proration is handled through Fastspring's API, and I have an API username and password set up. Is it that they see the full per month price, but that they only pay the difference when it comes to the current month? Any help would be welcome