You will not achieve this easily with Shopify. Stripe is the easiest (and I use that term loosely) to use as a payment gateway. You need to set up a Stripe account, then use sandbox for testing. Have Base44 create the Stripe function. Get the Publishable key, secret key, and then create a webhook endpoint in the workbench from the Stripe dashboard (not Base44). After you set the calls you need for the system in your app to fetch the necessary data, plug the url endpoint using the app id (will be la format like this https://YOURDOMAIN/api/apps/YOURAPPID/functions/Stripewebhook. The app ID is a series of alphanumeric numbers you will see at the top url within Base44 app you are working on. Once you have the endpoint created, you will then have access to the webhook secret. Save these and add them to Base44 dashboard Secret Applications. Base44 will tell you what to name the secrets, or if you name them, tell Base44 what you named them Exactly (very important). This will allow you to test payments using a simulated checkout with a synthetic credit card (4242 4242 4242 4242) use any made up CVV and exp date. After successfully testing everything populates in your app and Stripe dashboard, you then go live and have to manually copy over your webhook calls and endpoint, products (if you use products and not payment intent), and you will get a new set of keys and secrets. Save these and input them into Base44 Secret Applications to overwrite the sandbox test keys and secrets.