This article covers 

Can I edit a subscription to add an addon?

I cannot add non-recurring addons to a subscription

How to enable “All addons created” to be applied to a plan?

Summary

Addons can always be included while creating or changing/editing a subscription. To charge non - recurring add-ons, use the Add Addon option on the Customer/Subscription details page.

If you are are not able to see the add-on in the list, likely;

1. The Addon has been Archived (A deleted addon which is no longer in use except for the subscriptions linked to it. Cannot be reused for a subscription)
   
   

2. Restricted addon is enabled (Only specific addons are made applicable to a plan/subscription)

3. If you have allowed your customer to choose addons on the customer portal (please check if you have enabled the same under Configure Chargebee > Checkout & Self serve portal > Portal)
   
   
   
   

To manage add-ons, 

1. Go to Plan > Addons and click Add addons
   
   
   

2. For “Recurring” type addon choose Optional, Recommended or Mandatory and Click Add
   
   
   

3. For “Non-Recurring “ type addon choose any of the options listed below and click Add
   
   
   
   

4. Go to Plan > Addons and click Manage
   
   

   1. To set all addons to a plan select “All addons are applicable” and click Apply
      
      
      

   2. To set specific addons to a plan select “Restricted addons” and click Apply.
      
      

Related articles and Documentation

How to restrict certain addons for specific plans

How do I remove/change/add an addon to a subscription that already has one or more addon via APIs?