It is possible to upload invoices to an SFTP server and have them picked up from there automatically. An SFTP connection provides secure file system upload over a secure channel. Tradeshift provides a free account to a server where you can upload your invoices in a variety of formats.

In order to upload via SFTP, you have to activate the SFTP Uploader App in your Tradeshift account.

What is SFTP:

SFTP, the "SSH File Transfer Protocol," is not related to FTP, except that it also transfers files and has a similar command set for users. SFTP, or secure FTP, is a program that uses Secure Shell (SSH) to transfer files. Unlike standard FTP, it encrypts both commands and data, preventing passwords and sensitive information from being transmitted openly over the network. It is functionally similar to FTP, but because it uses a different protocol, you can't use a standard FTP client to talk to an SFTP server, nor can you connect to an FTP server with a client that supports only SFTP.

FTP over SSH (not SFTP) refers to the practice of tunneling a normal FTP session over an SSH connection. Because FTP uses multiple TCP connections (unusual for a TCP/IP protocol that is still in use), it is particularly difficult to tunnel over SSH. 

FTP over SSH is sometimes referred to as secure FTP; this should not be confused with other methods of securing FTP, such as SSL/TLS (FTPS). Other methods of transferring files using SSH that are not related to FTP include SFTP and SCP; in each of these, the entire conversation (credentials and data) is always protected by the SSH protocol. 

For a detailed documentation and guideline of FTP connection to your Tradeshift account please follow this link: http://tradeshift.com/integrate/sftp/