Royal Mail - The Field ItemQuantity

The error (Error Code:E1890 Description:The field ItemQuantity must be between 1 and 99999) from Royal Mail can be caused when the Quantity required for one or more of the items on the order is 0.
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If the Order came from Amazon you would need to Cancel and Delete the order then allow it to reimport. If the order came from any other Channel, you can change the quantity to the required amount and regenerate the label.