Find your product catalog in Settings :

If you have only one invoice item in Dashdoc, then we automatically fill the invoice item field with the invoice item available in the catalog.
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If you have several invoice items, Dashdoc will automatically fill the invoice item according to what has been applied on previous shipments.

Here, for example, Dashdoc has recognized that for my Belgian customer, the invoice item to be applied is "Transport excluding VAT":

You can always modify your invoice item in the drop-down list.
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A warning message warns you if an invoice item is missing from a shipment before you want to bill it.