Unable to add a one-time Add-on to a trial plan.
Unable to checkout with one time add-on.
One time or non-recurring add-ons, can be included on a checkout instance when immediate charges are applicable, that is, when the subscription is created in an Active State. Subscriptions which have a Trial or created in a Future state will not have an immediate charge that is applied to the customer, in such cases the system will not allow the inclusion of a non-recurring add-on.
Depending on your use-case you can choose to include Event Based Add-ons that can be applied at a specific stage of a subscription. Click Here to know more on Event Based Add-ons.
This is applicable when carrying this through the UI/Hosted Page/APIs.
For more information feel free to contact Chargebee Support.