It is possible to upload invoices to an FTPS server and have them picked up from there automatically. An FTPS connection provides file system upload. Tradeshift provides an account to a server where you can upload your invoices in a variety of formats.
In order to upload via FTPS, you have to activate our FTPS Uploader App on your Tradeshift account.
To learn how to configure it, please consult this page.