Aamco Transmission $499 charge to diagnose problem. Addison, Texas

My transmission began malfunctioning in Marsh Lane and I made it to Aamco transmission.  They charged $499 to diagnose the problem.  They then gave me a price of $4200 to repair it. 
My son and some friends referred me to a transmission shop in Plano who is long term and very honest.  Even after paying to have the car towed, and paying Aamco their $500, I saved $900.  When the transmission mechanic began removing the transmission he noticed several bolts that had not been replaced and a heat shield was missing between the exhaust pipe and a steering cylinder. 
I called Aamco about the missing part and he said he had a Honda undercarriage protector and to come by and they would check.  They put it on the lift and determined the part they had was not the right part.  The mechanic then claimed the part they had went to one of the cars in their lot, but he didn't know which one.  He made this statement after David, the manager, told me they never leave parts off cars, and whatever the car came in with, it left with.  I pointed out they were comparing a part that was left from another vehicle.  The transmission specialist didn't even know what the heat sheild was, or where it would go.
My shop showed me the bolt holes which were shiny and the outline where the plate went, which was also clean without rust.  It was obvious the plate was recently removed.  In addition, the exhaust has to be removed to install the plate and rests near the heads of the bolts, so the bolts cannot come out and the plate fall off.  Aamco just got mad and will not speak with me further.
$500 for a sloppy, incomplete job.  Now I have to pay to have another plate ordered and have it installed.  So much for transmission specialists!