Exactly that, they handle thousands of envelopes a day.
I had to do contract work at dozens of locations, I saw carriers sorting the suspected gift cards into a pouch they could go thru later on their route. Standard procedure at some offices, it was so blatant. The smaller rural offices, 2-3 people were perfectly honest for the most part.
They can tell how a card folds, how an envelope bulges, even the friction of bills vs paper,and whether the envelope says Hallmark on the back. USPS also gives them a 150 watt "sorting
light" they can hold it up to and make out serial# on the bills.
I wondered why they kept folding and squishing some pieces of mail, then I saw it being set aside.