My understanding is that "supplements included" should mean just that: supplements are included.
Thanks. So I'm not (that) crazy. I appreciate the validation.
I can't seem to make my links work in English, but
here is the link to Rome Mobilita in Italian where you'll see under the "
Tariffe taxi da e per gli aeroporti" that the fixed rate is "compresi i supplementi", as
iapetus quotes above.
I was quoting from the English-language version of the same webpage. I was actually looking for (and would have preferred) that site in Italian. (So, thanks again! ^

)
iapetus, your post and others I read here, make me happy to continue using my car service for €50.

Honestly, it's typically not an issue. The taxi cabs come quickly. Sometimes I've had pleasant conversations on the way to FCO. And I usually don't have a problem. But maybe I should bookmark the link above on my phone and show it to the driver when this comes up. Or just confirm it ahead of time. I could always threaten to report them, but who wants to go there? This is more of an annoyance than anything else.
That said, maybe you're right. I have a friend who keeps offering me to connect her with her car service. Perhaps I should take her up on it one of these days.