Export Supplementary Declarations (SDs) enable traders to provide full export information to customs on goods destined outside the European Union after shipment. Instead of submitting a full export declaration before export, authorised traders need only provide preliminary information in the form of a Pre-Shipment Advice. This is followed up with a supplementary declaration within 14 days of shipment. This could be helpful if you're exporting perishable goods and don't usually raise the export invoice until after goods have left.
You need to be authorised to submit SDs. Alternatively, you can use an authorised agent.