Indicate if there was a bound document in the envelpe
If you receive a (large) envelope with a bound document, TM won't scan it, by default. If there are individual papers inside, those will be scanned.
I was in a situation where those cover pages didn't make it clear whether there was anything else in the envelope. I expected something like 20 pages, but only saw 4. Turns out TM had silently not-scanned a bound document.
The suggestion is to clearly indicate in the item details if there are unscanned materials in the piece of mail.