Many Microsoft business plans have add-ons that can be purchased for our customers' subscriptions. Add-ons provide more features to enhance the "main" subscription's capabilities. We buy add-ons through our distributor, Pax8, the same way we purchase the main subscription.
While there are some add-ons that are linked to a specific subscriptions (traditional), add-on subscriptions for Microsoft 365 are generally sold as a separate (stand-alone) subscription and have their own expiration date. Unlike traditional add-ons, standalone add-on subscriptions aren't linked to a specific subscription. You manage a standalone add-on subscription the same way that you manage any other subscription.
If a customer is purchasing an add-on for an existing annual subscription, we should advise the client to purchase the add-on as a monthly subscription until the renewal date of the "main" subscription. That way, if and when the client decides to cancel the subscription at a future renewal date, the add-on can be cancelled at the same time.
If the add-on is added on an annual basis at a time not coinciding with the existing subscription's renewal, then when the customer cancels the existing subscription, they will be responsible for paying for the add-on until the add-on's renewal date. So it is usually best to line up the add-on subscription with the main subscription, even if this means paying a little more for a monthly subscription for a short time. At the the main subscription's renewal date, the add-on subscription can be switched to annual so that the renewal dates of the two subscriptions will be aligned going forward.

When a customer requests an add-on for a base subscription, it is critically important to explain (in writing) to the customer the value of aligning the dates of the annual add-on subscription and the annual base subscription. This should be done before the contract is issued.