Change Checkout Page

This isn't something you can change through the theme editor because the checkout page on Shopify is controlled by Shopify itself, not your theme. The only way to customise checkout layout and behaviour like this is with Shopify Plus which gives you access to checkout extensibility. If you're not on Plus the workaround is adding a clear notice on the cart page before they hit checkout, a short line of text or a banner that spells out exactly how shipping works for pre-order items and multiple packages. That way customers see it before they get to the compressed shipping summary. For the pre-order shipping message specifically you can also add it directly to the product page and the order confirmation email so it's communicated at multiple points without needing to touch the checkout itself.

Just quickly chiming in, OP I saw from your other post that you've already set things up using a pre-order app. Just a friendly heads up you can also customise the messaging shown on the product page (in the pre-order purchase option box, not just the product description), and add info as a line item underneath the pre-order product name in cart and at checkout. That + what u/VisioN0P covered above should hit pretty much all the touchpoints to set clear expectations with customers about the deferred payment upon fulfillment.

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The other comments are spot on – you can't touch the checkout layout itself unless you're on Shopify Plus. It's locked down for security and to ensure it's optimised for conversion across all devices, which is frustrating in cases like this but makes sense from a platform perspective. However, the root of your problem is communication, and there's a better way to handle it than just a banner. Look inside your pre-order app's settings for something called 'line item properties'. This is the proper Shopify way to do this. It allows you to attach a small piece of text directly to the product itself, which will show up underneath the item's name in the cart and, crucially, on the checkout page. It's much more direct than a general site message. You can set it to something unambiguous like: 'Please note: Shipping cost will be calculated and charged upon dispatch.' This way, the message is tied directly to the pre-order item they're buying, making it impossible to miss. It's a much cleaner solution.

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