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How do I allow users to add one-time or non-recurring addons in the customer portal

Yes, you can. In your Chargebee user interface, navigate to Settings > Configure Chargebee > Checkout and Self-Serve Portal > Portal. Enable the "Non-Recurring" (Pc 1.0)/"charges"(Pc 2.0) option to allow customers to add, remove or change the addon quantity.

For Product Catalog 1.0 customers

For Product Catalog 2.0 customers

You can choose the settings in such a way that it fits your business use case. 

For addons and charges to appear in the Self-Serve Portal, you must enable the option Display in Self-Serve Portal while creating addons and charges.

If not enabled already, you can use the Edit action to enable it.

To enable it, click Product catalog > Addon (or) Charge and select the addon or charge which needs to be displayed in the portal. Under the Customer-Facing Info section, enable Display in Self-Serve Portal.

Further, there are some scenarios during which you cannot add Non-Recurring addons to the portal:

• Non-recurring addons or Charges cannot be selected in the “Allow customers to add addon” option if the changes are set to happen at the end of the term. They are allowed only if the changes are set to reflect immediately.
• If the plan is addon restricted, then a Non-Recurring addon or charges cannot be applied. This is because these add-ons are event-driven. 
• Non-Recurring add-ons/charges can only be added to Active subscriptions. It cannot be added to Trial and Future subscriptions.

Read about one-time payment checkout here.