I understand that it's the employees sorting and delivering the mail, but you need to look at the bigger picture and not immediately jump to blame the product of a broken system.

Management sets the working conditions, policies, and provides resources (or sometimes fails to).

Mistakes can often be traced back to issues like understaffing (possibly due to low wages), lack of proper training, or unreasonable expectations - all of which are decisions made by management.

If management practices improve, the overall performance and accuracy of the workforce will likely improve as well.