TRX orders have a "Messages" tab available to all associated users.  
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Posting a message is quick & easy.  Open the Messages tab and enter your message into the text box and click "Post".  
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You can also attach documents using the attach/paperclip icon
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The "Messages" tab isn't just for messages & attachments, it also keeps a log of activity on the order.  Notice in the screenshot below, the messages tab displays when the order was submitted as well as any posted messages.

If you have any questions, you can always reach us via chat (below) or customersuccess@trxchange.com