It's my understanding that with a first class conventional size US commercial envelope that is of a size and shape to fit into the regular stream going into an automatic facer canceller (Mark II or other), that the facer canceller would just look for a stamp with a tag and sensing one, then it is moved along in the mail stream without human inspection of the stamps' denominations.
If the address cannot be completely read by machine, it is then photographed for a human operator at a distance who reads the address and keys in the necessary data so that it gets sprayed with the zip+4 address coding when it is put back into the sorter a couple hours later. The spray code then is used throughout the rest of the letter's journey through multiple sorting machines across the country until it is in the local carrier's bag or tray. https://www.endicia.com/en/tools-re...cation-marks
gives an interesting (to some like me) illustrated tour of the path of letter-size correspondence. He concludes that the delivery carrier might spot short postage, but it is unlikely.
Packages and anything that requires manual handling is another matter. But there one has to ask how much pressure is upon a desk or back office clerk to prevent underpayment? Are they at all motivated to distinguish the different values of the 3 G stamps or various Madonnas with child?
I like that cover Don.