You could have asked to see the old parts when you picked the car up. I don't know what you expected to "feel." The improvement provided by most performance parts can't be felt. Usually you need to time the car on a test track or hook it up to a dynamometer to see a difference. If your car has a tachometer you might be able to see a difference in RPM at a particular speed if you have a record of what speed versus RPM was before the parts were installed.

You can have a different mechanic disassemble the transmission and validate the aftermarket parts were not installed (with pictures and a signed affidavit) then sue the original mechanic.

I think it's far more likely that the parts you bought for your tranny are snake oil. Most "performance" parts are.

If you're really concerned, open up the transmission and look--or pay someone to do it for you.

what kind of transmission . . . what kind of "upgrade" parts . . . .what changes did you expect ???

I bought a $1k worth of upgrade parts for my transmission. My mechanic told me he installed them but I don't feel a difference and I know he lied to me.

He's a certified mechanic that works out of his garage.

I've been to him 4 times in the previous 5 years and never had problems until now.what can I do about this since it's not a business shop?