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Prepare a spreadsheet that will contain a list of products. The first column always has to be the the line item ID (sku). If you want to import other attributes, their order must respect the order of the columns in the table.
Copy the content of the spreadsheet to the clipboard (Ctrl+C).
Go to the items Items tab and click the Import items from clipboard button.
Paste the spreadsheet content from the clipboard to the text box and click Import.
Select whether you want to append the existing list of products or replace it with the imported list.
Verify that the products have been imported correctly.
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