I am aware that AutoEurope is not CA based. I don't now how "doing business in CA" applies to an internet business but the CA law does specifically address "computer displayed" quotes. Theoretically one could get a judgment against AutoEurope which would be valid if they ever tried to "do business in CA" (or even perhaps if they were bought by an entity that did that).

I'm hoping that the CC company will see reason and do the right thing. One time previously that this happened to me I had a good experience with American Express. I don't know how Barclays will handle it.

Reading further I think the same CA code section does exempt Hertz provided Hertz can demonstrate that they informed AutoEurope of the fees:
"(B) If a person or entity other than a rental car company,
including a passenger carrier or a seller of travel services,
advertises or quotes a rate for a car rental at an airport where a
customer facility charge is imposed, that person or entity shall,
provided that he, she, or it is provided with information about the
existence and amount of the fee, to the extent not specifically
prohibited by federal law, clearly disclose the existence and amount
of the fee in any telephonic, in-person, or computer-transmitted
quotation at the time of making an initial quotation of a rental rate
and at the time of making a reservation of a rental car. If a rental
car company provides the person or entity with rate and customer
facility charge information, the rental car company is not
responsible for the failure of that person or entity to comply with
this subparagraph when quoting or confirming a rate to a third person
or entity."

(Though this exemption only seems to apply to mis-quoted Customer Facility Charges?).