Sixt is apparently big in Europe, and they resell Alamo in the U.S.. On
Sidestep, they show up much cheaper than Alamo, and the rate says that
it includes tax, but I wonder if it really includes all the fees that
Alamo (and other agencies) charge for on-airport rentals ($162 for the
rental I'm interested in).

Anyone use them in the U.S.?

Presently I have an off-airport Avis rental for $384, and the Sixt price
is $421, and the difference in the hassle would be worth the $37. Alamo
has a price of $748 for the same rental that Sixt advertises $421 for.