How to grant free months to an auto-renewal subscription (iOS)?

I have an iOS app with different auto-renewal subscription possibilities, and I want to offer it's user the chance to earn a free month when they invite friends to the app.

I know how to handle the scenario where I'd give a free month to a user that's not already subscribed, but I'm wondering how to handle a user that's in the middle of a year-long paid subscription and should receive a free month..

1. Promotional Offers

This option allows you to add a free or discounted price for a period that you specify and it will be executed before the next payment cycle. You can use this feature starting with iOS 12.2.

Apple Documentation:

You can provide lapsed or current subscribers a limited-time offer of a discounted or free period of service for auto-renewable subscriptions on macOS, iOS, and tvOS.

Some useful links:

• Implementing Promotional Offers in Your App
• Set up promotional offers for auto-renewable subscriptions
• Subscription Offers Best Practices video - WWDC 2019.

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2. Offer Codes

Apple Documentation:

Offer codes can help you acquire, retain, and win back subscribers by providing a subscription at a discount or for free for a limited time.

Read more

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3. "Extend a Subscription Renewal Date" endpoint:

Potentially simpler alternative that is not really suggested to use for this use-case. It has certain limitations though:

• you can move the renewal date for a customer’s subscription up to 90 days into the future for each extension
• you can extend the renewal date twice within a year (365 days) per customer

The following types of subscriptions aren’t eligible for renewal date extensions:

• Subscriptions in a free offer period
• Inactive subscriptions in a billing retry state
• Subscriptions in a grace period state, with an expiration date in the past
• Subscriptions that have already received two renewal date extensions within the last 365 days

Read more