This is a bit tricky, but if your CSV is not too long, you could encode it in base64, send to the webserver as a GET parameter and then, in the server side, decode and store it as a CSV file.
If the file is too big to do that, you have to use other method, like the java applet pointed by @D.Schalla or even install and configure a FTP server, and make the Point of Sale app uploads the file there.
Other alternative, specially good if you cannot modify the sale app, is to install a web server in the client side and write a small php script to handle the upload process. In this way, the sale app could call a local url (something like: http:// localhost/upload.php) and it's this script the one in charge to upload the file which can be achieve with a classical HTTP POST, a FTP connection or any other way you can think about.