Now it drips about 5-6 drops a night. (I placed a sheet of paper underneath the car to check). Is this due to overfilling or did they mess something up?

/twospoons

 

just a quick guess, but they probably didn't replace the crush washers and they're leaking.

 

Or they didn't clean up real carefully around the fill hole. There's a ledge right below the fill hole that can hold quite a bit of fluid. Make sure that is cleaned thoroughly before looking deeper.

 

Or they didn't remove the old crush washer and added a new one and now there are two. Depending on the experience of the tech, he might not have noticed that the old one was fused to the trans or to the drain plug. It can be hard to tell that the washer is still attached.