# Set up an add-on with Rebilly Instruments This topic describes how to set up an add-on for use with the Rebilly Instruments JavaScript library. - The add-on feature is in beta and active development. This means it may contain bugs and instability. - To use an add-on, it must be used together with the `items` property. Important This topic assumes that you have already completed one of the [Get started guides](/docs/dev-docs/get-started-integrate-payments), and are looking to customize or extend the functionality. ## Add-ons setup The add-ons will show up at the bottom of the confirmation screen. Preview div iframe JavaScript ```js import RebillyInstruments from '@rebilly/instruments'; RebillyInstruments.mount({ organizationId: 'test-org-2019-12-07', publishableKey: 'pk_sandbox_WcUZLs3xwB7dubewna3zJA75wdMBEHeWHOyQww8', websiteId: 'ach-example', apiMode: 'sandbox', items: [ { planId: 'my-plan-id', quantity: 1, }, ], addons: [ { planId: 'my-addon-plan-id', quantity: 1, }, ], }); ``` HTML ```html

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