In the new version of the checkout pages, you can list the add-ons that you want to provide to your customers on the checkout page with the help of Grab Code option. This helps you in copying the respective code for specific add-ons in your webpage and display the add-ons during checkout.

This option can be found in the Plans detail page in your Chargebee account. You can copy the Plan code and the Add-on code and embed it in your webpage. This helps in allowing your customers choose additional products along with the plan from your webpage.

If you're using Chargebee's Customer Portal and you want your existing subscribers to add/remove recurring add-ons from their subscriptions, they could simply login to the portal and do it. You can specify what operations can be done through the portal under Settings>>Configure Chargebee>>Checkout & Self-Serve Portal(under Customer-facing essentials section). Enable the options to add or edit the add-ons.

 If you'd like to attach more than one plan to a subscription, you could create a $0 plan and associate multiple recurring quantity-based addons {one for each product}, as we support only plan per subscription only.

Note: Chargebee provides add-ons to help you manage your customer's additional product needs. The additional products can be modeled as a recurring or a non-recurring addon depending on whether it will be a one time purchase or a recurring charge on top of a subscription.

If you're using the V2 checkout pages, check this article.