Here is a step-by-step guide for creating add-on services that can be linked with your main product for subscription purchases.
Problem Statement - The client sells different diet plans, now they want to sell recurring services to the customer along with the main product. Eg: Online coaching of price $30 so customers will be charged $30 every month for this classes once they opt for a subscription.
Create a product on your Shopify admin panel named as Online classes.
Now create a monthly subscription plan on the Recurpay dashboard and under the products section choose the newly created product.
https://as-fitness1.recurpay.com/plans
Now on your main product page show the addon product checkbox selection and fetch the subscription plans using our Plans API.
Whenever a user adds a product to a cart on click of add to cart button, use add.js of Shopify to add the main product and the addon product with the selected subscription plan id if the checkbox is selected.
Please ensure only the addon product will be added as a subscription product.
https://shopify.dev/api/ajax/reference/cart#add-a-selling-plan
That's it, so now whenever a customer goes to checkout they will have a recurring product with their main product. This add-on product will be renewed every month and the customer will be charged accordingly.