I would be wary of any service that quotes you a higher rate than is listed on their web site. This may sound like common sense, but three services did this to me, claiming that the difference was for tolls and port/airport charges. Can you say "bait and switch"? One service had the audacity to tell me that they were being "upfront" with me by quoting me a fare in resopnse to an email that I sent that was $25 higher than the fare listed on their web site. When I pointed out that if they wanted to be upfront with their potential customers they should include the extra charges in their web-listed fares, the response I got was that 1) "every company does it this way" and that 2) nowhere on their web site does it say that the company will be responsible for tolls and port/airport charges. True for 2, but also nowhere on the web site does it say that these extra charges even exist, or that they do exist and the customer is responsible for paying them above and beyond the web-listed fare.
I found a company that walked the talk. The fare listed on the web site was the bottom line, except for the gratuity, which should be left to the customer's discretion, anyway (one company includes an automatic "gratuity"- what happens if you're not happy with the service?).
Buyer beware.