Most Shopify merchants focus on service selection and carrier rates when thinking about UPS shipping costs. But the dimensions entered at the time of label creation are just as consequential, and mistakes here show up as invoice corrections after delivery, not upfront warnings at checkout. Incorrect UPS package dimensions lead to dimensional weight miscalculations, missed Additional Handling surcharges, and billing corrections that arrive weeks after a shipment has already been delivered. This guide breaks down exactly how UPS uses package dimensions to calculate rates, what happens when those dimensions are wrong, and how to build accurate measurement into your Shopify shipping workflow. Billable weight is not always the same as actual weight; it depends entirely on the relationship between a package’s physical dimensions and how much it actually weighs.