I'm looking at a rental which is substantially cheaper if prepaid. But there's a decent chance I'll want to return the car early.

On a postpaid rate this would be straightforward. Let's say my rental reservation prices as 1 week at $x plus 2 extra days at $y. If I return after 1 week and one day, I pay x+y.

What happens on a prepaid rate? No refund for early return?

The Hertz.com booking screen presents a table that purports to cover "Cancellation/Change/No-Show Rules". But it actually only discusses cancellation and no-show. Contrary to the table heading, the table doesn't say a thing about change rules or fees!