I was able to MODIFY an existing reservation to half-way solve this issue:
Originally was a 10:30am pickup and then 10am drop off a few days later.
Schedules changed, so I modified PU to 8:30am and 11am drop off - Hertz added a $29.64 charge for the add'l 3 hours of rental time.
Did this change online, made sure that "keep same rate" box is checked. I have also called in the past to PC line and had them change it.
Daily rate was like $90-ish, so certainly a bit of an up-charge relatively speaking, but $30 beats paying for a full extra day when all I needed was a few extra hours.