With tradeinterop you can send an unlimited number of attachments, up to a maximum of 5mb per attachment (with a total of 10mb).
When you create an invoice in the tradeinterop portal, you can easily add attachments by dragging them from your desktop to the browser screen where the portal is open. You can also add attachments by clicking in the box with the blue border.
Once you have added files, they will be displayed under the invoice data.
