With a one-time payment, you receive a limited amount of items or access to a catalogue for a limited amount of time. Once you’ve used up all items or the time of access is up, you will need to make another purchase.
To fill up available texture and render slots you will need to make an additional purchase.
You can do that from the Profile section by clicking on the available amount of textures or renders. Once the payment is charged and confirmed you will receive a top-up of purchased items.