Orders sent to Royal Mail Click & Drop are importing only product weights, not including packaging/box weight, leading to underweight shipping labels unless adjusted manually. Shopify shows correct calculations on its side, but the integration appears to pass through product weight alone.
One user reports the same issue and asks if a fix exists. A temporary workaround is to add the package weight into each item’s weight, which works for single-item orders but overstates weight for multi-item orders.
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Outcome/status: No confirmed native fix or setting was provided in-thread. The discussion remains open with third-party apps suggested as the primary workaround.
Hi
My products are filtering to click and drop but I have a problem where it is not adding the box rate to the postage weight so we have to manually change the entries. Is there anything I can do so the correct weight pulls through to royal mail please? Shopify is calculating correctly but only the product weights are pulling through so if we just printed the labels it would be underweight
Thanks 
Hi Pommad
I have exactly the same problem. Did you ever find a solution?
I can only think of a workaround that works for single product orders, which is adding package weight to each item weight.
But this would then add far too much weight for multiple item orders.
Kerry
As one suggestion, have you tried a third-party app to add your packages (and thus package weights)? There are a few options in the Shopify store like Starshipit (our solution) and Boxify. It may not be what you’re after but possibly worth investigating.