I’m building a Shopify store that sells made-to-measure curtains. Every order’s price must be calculated from the shopper’s width and height inputs, then carried all the way through to checkout without relying on any front-end trickery or draft-order workarounds. In other words, the figure a customer sees on the product page has to be the exact figure that lands in the cart and is enforced at payment. What I need: • A clean, production-ready implementation—theme code, a bespoke Shopify app, or a hybrid approach—that watches the two dimension fields, runs the formula I’ll supply (price per square metre plus optional extras), and injects the true line-item price. • Complete preservation of Shopify’s native discounting, taxes, and shipping calculations. • Compatibility with Online Store 2.0 sections, Ajax carts, and the standard checkout so nothing breaks when Shopify updates. • Clear setup notes so I can tweak the rate table later without touching liquid or React if possible. Acceptance test 1. Enter arbitrary width/height, see price update. 2. Add to cart; cart drawer and cart page show identical value. 3. Proceed to checkout; same value appears on every step and on the final order in admin. If you’ve already solved similar “price-on-the-fly” challenges—especially for custom blinds, fabric, or printing—let me know what stack you used and how you’ll lock the price server-side. I’m happy to provide theme access and a staging URL right away so you can get started.