Withfriends add-ons are added manually by the merchant when setting up each tier. Add the price, and the charges will either recur with each shipment, or automatically end after one shipment. Feature isn't available on all accounts, so inquire with support@withfriends.co for setup.

Subscribers can access these add-ons in their subscription dashboard, as pictured below.

If you're using the add-ons feature, the subscriber will see any selected add-ons listed in their Upcoming Shipment notification, along with some simple marketing of the currently listed add-ons. You can update the add-ons at any time as inventory is available.