While a subscription is an auto-renewable payment (that will be charged off your card every month/year), renders are a one-time purchase.
With a one-time payment, you receive a limited amount of items. Once you've used up all items, you will need to make another purchase.
You can do that from the Profile section by clicking the icon in the upper right corner. You'll need to tap on the "Renders" section to access a purchasing screen.