I am unable to append it as per your feedback.
why?
what happens?
wrong result?
error?
nothing?
it works correctly on my test
no difference to your code except the mail item object name
for your exact code try
Reply.Body = "Dear _____," & vbNewLine & vbNewLine & "The order has been modified as per your request." & vbNewLine & vbNewLine & "Change Detail: (Item Name & quantities updated)." & vbNewLine & "Reason: (as mentioned by the CS dept)" & vbnewline & reply.body
but you may still need to make some changes, to obtain the desired results