At any point in the month, if you need additional credits to purchase a new item, you'll be able to make a one-off credit purchase. Just click download on the item of your choice and then select 'One-time purchase.' You'll then be shown the price to purchase the balance of credits required to download the item.
Please note, that only credits purchased as part of your membership will include a 20% savings. All one-time credit purchases will be at full price.