Saving Sent Items in Delegated/Shared Mailbox

Bear with me as I try to explain this one.  On SBS 2011 server with Exchange 2010, I have an "accounting" mailbox that several employees have full access to review email, etc.  Obviously, when an employee replies to an email that was sent to the "accounting" mailbox, the "sent item" ends up in the user's Sent Items folder.  However, the users would like a copy of the Sent Item to be placed in the Sent Items folder of the accounting mailbox - or even some other folder I can create in the accounting mailbox - so that those who share the accounting mailbox will know if someone has responded to those accounting messages.  I pondered perhaps something like a Transport Rule to read an outgoing email, then to route a copy of the sent email back to the accounting mailbox, but I don't think that's going to do what they need.  User's rules in Outlook won't work from what I can see due to all the potential variables of each message (recipient address, subject line, etc.)  Does anyone know of a way to accomplish this that I may be missing.

Giving Send As access I guess in an option.   Then, I could probably set up some type of transport rule, however there would require some manual selection to send as - which is open to error.  Also, I'm reluctant to assign Send As permissions in this way.

It's similar to when mailbox delegates delete a message from a mailbox to which they are delegates wherein the deleted copy goes to their on Deleted Items rather than to the Deleted Items of the delegated mailbox.