As with anything else in life.. if it sounds too good to be true, it probably isn't. Be careful with guides offering deeply discounted prices. I can't offer you any links to sites that might have reviews of this company, maybe you can just Google their name and find one???
Also, you are more than welcome to stay here and ask for more generalized help. There are several members here that have vast experience in Europe and Italy.
Can anyone recommend a reliable tour guide for the Amalfi coast?
You also can just wait and bargain with them at the pier. You end up paying by the car load so if you can find another couple that want to do a tour you might be able to reduce your per person cost be touring together. When we went our friends did this and got a tour guide to drive them for the day for $300. Which is not too bad. That was before the economy tanked so you might be able to do better than that.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013