Carriers price size as well as weight
Carriers bill on the greater of actual weight and the weight a shipment's volume implies, and heavy or bulky goods move into freight services with their own rules. A catalogue that stores a price and a photo but not carton data cannot quote any of that.
The data usually isn't there
Supplier feeds rarely agree on units, and many omit carton data altogether. On Shopflooring, where twenty-plus supplier price lists were normalised into one schema, density and weight are calibrated per category so freight quotes stop being guesses.
The quote has to land before checkout
Shopflooring rates freight in real time before checkout, and large-ticket orders can be financed through Klarna, Afterpay or Affirm via Stripe — which only works if the freight figure is inside the total when the customer is handed over.
Some loads should not be rated live
Oversize pieces and awkward mixed loads are better handled by an honest "we will come back to you with a number" than by a plausible figure. Norpex is built around that: quote requests route by product category and vendor, beside one checkout for what can simply be bought.
The full write-up is in our post on freight.