Add weight or page estimation
I browsed through ideas and several mention this already. One resulted in the FlexScan idea which seemed to not address the user's request. Another makes the request for bound material and CDs. Another offered a bunch of suggestions related to determining what's in the package/envelope. There seems to be a decent amount of interest on user's parts to be able to better assess what is in the envelope before opening it.
My suggestion would be to add some type of weight field or at least a page estimate or page range estimate. I don't know your workflow but I'm assuming since a barcode is on each piece that means a human is entering some meta data as the barcode is generated, so this would be one more piece of meta data entered. Obviously weighing every item requires more time and costs (buying scales, etc) so maybe implementing as a "best judgement of what's inside" and have a drop down for the envelope handler that's something like 1-5 paper, 6-20, 21-50, CD, credit card, etc.