Adding an add-on to a subscription can be done at any time, either manually in the app, or by the customer using the account management modal. How the add-on is treated and charged in the first cycle depends on the set-up of the plan.
There are three options you can choose from when setting up the billing date in your cycle:
Do not pro-rate
Do not charge until the next cycle
Pro-rate based on days remaining
If a customer is subscribed to a plan and cycle that is set up to pro-rate or not to charge until the next cycle, when the add-on is added to their subscription, the price they're charged for the add-on (and the units of the add-on they receive if the add-on is unit based) should be pro-rated.
For example, if you charge your customers on the 1st of every month and a customer decides to join on the 15th, the number of units included in the allowance would be pro-rated based on how many units the customer was due to receive each cycle, alongside how many days they have left in their current cycle. So, if the customer was due to receive 10 units each full cycle, but they joined at exactly halfway through, then they'd only receive 5 units to use until the end of the billing cycle.
On their next bill date, however, they would be charged the full amount and receive all the included allowances for the next cycle.
If you have your cycle set up to charge the full amount at the point of subscription, regardless of when they sign up (Do not pro-rate), the full amount of the add-on will be charged when the add-on is added.
Pricing models
Pro-rating works for all pricing models of add-ons as well as all pricing models of plans. When you're applying the add-on that has anything but a flat fee pricing model, you or the customer will be asked to input the units of usage, and then we'll calculate the price via pro-ration after the units have been selected. Then you're good to go!