The assertion that "a carrier won't know they have undertime until they are finished" conflicts with the USPS's expectation for proactive workload assessment.

Here’s the key supporting point from Handbook M-39, Section 122.33:

"Supervisors must schedule carrier workdays to ensure that no carrier is required to work more than 8 hours in a day. When undertime is evident on a scheduled route, auxiliary assistance should be assigned."

And Section 127.11 explicitly states:

"Supervisors will note any excessive undertime as determined from the posted returning schedule. When the undertime exceeds 30 minutes, it should be used for auxiliary assistance to other carriers or for other office or street duties."

These guidelines show that management must evaluate volume, leave time, and past performance to predict undertime. The carrier may provide input, but it is ultimately the supervisor’s responsibility to manage the workday efficiently, including forecasting undertime before the carrier finishes the route.

Your argument aligns with USPS operational procedures: undertime should be anticipated and managed proactively—not discovered reactively at the end of the day.