Our payment processing runs on a "subscription" based system. When you sign up for a paid plan, or purchase add-ons, you create a "subscription" for that service within your billing portal. Subscriptions are billed on a monthly or yearly basis, and they recur on the same date of the month/year as when you originally created the subscription.
So, for example, if you sign up for a monthly Agency membership account on the 14th of the month, then you will always be billed on the 14th of the month. If you sign up for a yearly Freelancer membership on June 22nd, then you will always be billed each year on June 22nd.
Add ons are now rolled into the existing membership subscription, and are billed on a pro-rated basis the first time around.