How the price reaches checkout
The engineering behind Print Price Engine's custom prices, covering cart line properties, the Shopify cart transform function, quantity-change safety, and one-click reorders.
"A calculator that shows a price" is easy. Charging that price at Shopify checkout, correctly, even when the customer edits their cart, is the hard part, and it's what this app is actually built around.
The flow#
- The calculator quotes: dimensions, quantity, and options are priced live by the engine.
- Add to cart attaches the maths: the cart line carries the customer's choices as readable properties (width, height, options) plus hidden pricing data: the base unit price, the fixed-cost portion, and the full discount ladder.
- A Shopify cart transform reprices the line: a native Shopify Function (compiled to WebAssembly, running inside Shopify's checkout infrastructure) reads that data and sets the line's actual price to the calculated amount.
- The order arrives production-ready: dimensions, options, and the artwork link sit on the order's line item for your team.
Why a cart transform, and not tricks#
Common workarounds have sharp edges: hidden 1-cent variants pollute your catalogue and analytics; draft-order flows break the native cart; JavaScript price displays don't survive checkout. A cart transform is Shopify's first-class mechanism for exactly this. The price adjustment happens inside checkout itself, works with any theme, and can't be bypassed by editing the cart URL.
The quantity-edit safety net#
The subtle failure mode of calculated pricing: quote 50 units (30% volume discount), add to cart, then edit the cart down to 10, and keep the 50-unit price.
Not here. Because the entire discount ladder rides with the cart line, the transform recomputes the discount from the live cart quantity at checkout:
final unit price = (base − fixed) × (1 − tier discount for current qty) + fixed
Quantity up? The better tier applies automatically. Quantity down? So does the correct, smaller discount. Your fixed costs are excluded from the discount either way.
What the order shows#
- Readable properties: Width, Height, Quantity, every custom option, exactly as chosen
- Artwork link: the uploaded file, downloadable from the order
- The engine's internal pricing properties stay hidden (underscore-prefixed), so order printouts stay clean
One-click reorders#
Past custom orders show a Reorder action in the customer's Shopify account. Customers can adjust the quantity and re-upload artwork; the configuration and pricing carry over. Repeat print business, the best kind, without re-entering a single dimension.
The cart transform must be activated once from the app's dashboard (step 4 of Getting started). If quotes look right but checkout charges the base product price, activation is the thing to check.
Related guides#
- Quantity discounts for the ladder the transform enforces
- Troubleshooting for checkout price issues
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