On slow and laggy 'ol Doomhammer, you can see that it can take quite a while for the server to send the data for the invoice (same actually with sending the data for the text of the message) even though you can see the item/gold to click on it. And you can click on the item to "take" it long before the invoice contents shows up, although AFAIK you don't actually get the item until the data has arrived from the server first, and then the item is delivered to you (even if you clicked on it 5 minutes earlier).
So at the time you click on the item, the invoice data may not yet be available, but it should be available by the time the item is delivered from the inbox.
Hope that helps you get a handle on it
-- Cirk