Promotional Product for Subscription

A custom addon for Woocommerce Subscriptions, which add a promotional product to the currently active user subscription and remove it after regular payment. Billing happens through the saved payment option on the renewal date.
For creating a promotional product that can be added to the subscription in option “Product data” in admin was added additional tab “Upsell product for subscription”. The new tab contains the options that do product an upsell product for subscription and the field that allows changing the button name for adding product to subscription.