The best option for implementing a custom variable pricing system on Shopify, without resorting to third-party apps, is to create a custom app. This will allow you to integrate your specific pricing logic seamlessly into your store. Here's a step-by-step guide on how you can achieve this: Create a Private App: In your Shopify admin, navigate to Settings > Apps. Scroll down to the bottom and click on 'Manage private apps.' Create a new private app to get the API credentials needed for communication between your app and Shopify. Set Up the Backend Logic: Develop a backend system that can receive user input (e.g., length in meters) and calculate the price based on your specified formula (Price = Initial_Cost * Length / 4). Use the Shopify API to create a draft order with the calculated price. You can refer to the Shopify API documentation for details on creating draft orders. Integrate the Frontend: On your Shopify store's frontend, implement a user interface that allows customers to input the relevant value (e.g., length in meters). Use JavaScript to capture this input and send it to your custom backend for price calculation. Handle the Checkout Process: When a customer is ready to make a purchase, use the Shopify API to convert the draft order into a finalized order for seamless checkout. By following these steps, you'll have a tailored solution that meets your specific pricing requirements without relying on third-party apps. Keep in mind that maintaining and updating your custom app will be your responsibility, so periodically check for updates to the Shopify API and make adjustments as needed.