Toyota Repair: Engine Check Light, toyota camry v6, charcoal canister

Question
I have a 2001 Toyota Camry V6, 92,000 miles. In Sept 2006 I accelerated onto the HWY my engine sputtered and stalled and my engine check light came on. My manual indicated that I should take my car to a repair shop immediately.  On 09-07-06 I took my car to a Toyota repair dealership where they performed a Scan Test $74.50 and said I had a code P0446 which is a Charcoal Canister Failure so they replaced it and reset the check engine light and cleared the code the cost $554.28.  In November 2006 while entering the HWY at an accelerated speed my check Engine light came on again I took my car back to the repair shop on 11-03-06 and a code P0304 came up indicating that my #4 cyl had misfired they replaced it and cleared the codes for $428.35. On 11-24-06 while accelerating onto the HWY my check engine light came on a third time I called the service manager at the Toyota repair shop and explained how frustrated I was and bewildered by this reoccurring problem and the money I've spent and I still had  the same reoccurring problem. He said diagnosing the check engine light was hard and suggested I bring my car back to the shop which I did on 11-28-06 there diagnosis this time was #6 cyl misfire, they did not charge me the usual scan test fee of $74.50 but said they were still not sure how to fix the problem.  So they switched the coil on the #6 cyl with the #2 cyl and the check engine light went out, they figure if the # 2 cyl misfires then the coil is the problem. I will test it tomorrow when I accelerate onto the HWY and if the problem reoccurs I will take my car back to the shop. My question is this; have they replaced parts that really were not the problem and charged me needlessly?  

Answer
sorry for the delay, Ive been at Lexus training for 2 days.

Well the emmisions code would NOT make the vehicle stumble or stall.
You may have only needed the bypass vsv on the charchol canister ($60)and not the entire assembly.They should have active tested the bypass vsv. so that could have been a $180 repair.
The coil should be an hour labor and about $100-125 for the coil, I dont see where that should add up to $428.35, but to be fair, i would need to see the receipt.

Its a simple engine with good diagnostic capabilities, They shouldnt have to take 4 shots at it, at your expense, to fix it.

They seem to have no problem cashing your check!

Mike