When a user purhases a subscription or one-time purchase. We want to add a free item to the basket. How do we do this?
The best way to add an additional item to the cart when a customer adds a specific product to the cart is with the Cartbot app (https://apps.shopify.com/cartbot-auto-add-to-cart).
Then, to apply a discount when buying this item together with your required item, simply create a bundle in Bundler app, set a 100% discount on the desired product (by selecting the “Set discounts on products” option) and save the bundle.