On Tradeshift, invoices can be "flipped" into credit notes with the click of a button. To do so, find the invoice in the Document Manager App. Click on the dotted area (1), and then click on "Create Credit Note" (2). A credit note draft will be created, addressed to the same recipient, and with all the same contents as the original invoice. You can review it and add a message before sending the credit note.

Read more about this process starting page 48 of this document.