this is a little convoluted to set up:
add a column to the eCart object for the uploaded file.
add the add to cart behavior to add the item to the cart and set the price
add a file field to the add to cart button form, then add the file upload server behavior.
after adding the upload server behavior, edit the add to cart button server behavior, and bind the upload file column to the Server File Name setting from the Upload behavior.
you will also want to add a column to the order details table, and edit the store order details behavior to bind the upload file database column to the cart column.