Most plans include a monthly download limit. Once you use it up, you can keep downloading — extra assets are charged individually and appear as a separate invoice at the end of the billing cycle. The Ultimate plan has unlimited downloads, so overages don’t apply there.
Overages are not available for PayPal subscriptions — once you hit your limit, downloads stop until the next billing cycle.
You can check your remaining downloads in the Products tab of your account. If you see charges you don’t recognize, contact support with the invoice details.