How can I tell if a visa agent is trustworthy?

I'm about to apply for a visa for an upcoming trip to China, so I'm looking into visa agents - those companies that will accept your passport and application documents by FedEx or UPS (etc.) shipment, submit the application on your behalf, pick up the passport with visa and ship it back to you. Because the process involves parting company with my passport, I want to be very certain that whatever visa agent I use will keep it secure and return it in the time frame that they say they will. Plus, of course, I also would like to know that I'm not handing a couple hundred dollars off to a scam artist (though I know this is quite unlikely). How can I tell whether a visa agent I'm considering is trustworthy? Is there anything like an accreditation procedure or a list of visa agents which are known to be legitimate?

The reason I wonder about this is that I have some concerns about one company which was suggested (if not necessarily recommended) by a colleague. On their website, they say that a visa application at the Chinese consulate in New York costs $140 plus a $60 service fee for 10 business day processing, or $160 plus $85 for 4 day processing; however, when I emailed them to confirm this, they said that $140+$60 was for 15 day processing and $160+$85 was for 10 day processing. Also, their web page says that the visa requires a confirmation letter from my US employer (in my case, my university), but this is not listed as a requirement on the consulate's web site. I know these are minor things, but it's enough to make me wonder why these inconsistencies are there, and whether they're signs of something I should be concerned about.