In my actualy experience, the surcharge is $2 or $3. You can decide whether that
counts as hefty, in the context of everything else you pay for a car rental. ...

Perhaps it depends how many tolls you go through, and whether they
consolidate the bill. When I was in a hire car in USA last summer we
went through maybe five or six each way per day on a "road trip". We
were paying cash, but I was aware they had cameras to send bills to
people using the transponder lane without having a transponder.

In the US, some toll highways that have ETC also take cash, some don't. If
you blow through a transponder lane on the Garden State Parkway without a
transponder, you'll get a hefty fine in the mail, because that's a violation.
People without transponders are supposed to use the cash lanes available at
all of their toll plazas.

On the toll highways in Miami, on the other hand, the "toll plaza" is just a
gantry over the road. You can't pay cash. So in that case, what you get in
the mail isn't a violation, it's just a bill, and the surcharge is quite low.
I gather that in Florida, Sunpass has an arrangement with rental car companies
to report the tolls on their cars in close to real time so they can often
add the tolls to your bill when you return the car.

R's,
John