When I mention something about the build quality of my car my local mechanic always replies with 'it's an alfa you know!'. I have a feeling mechanics hate to see an Alfa coming through the door as there aren't many of them about and therefore the mechanics have little experience with the brand. With a medium scale brand such as Alfa it is always best to take your car to a specialist who knows the car and doesn't mind their foibles.
Unfortunately nowadays labour is so expensive so that any work, major or minor, to gearboxes or even engines to a degree, tends to seem extortionate. The garage will more than likely want more money to investigate which is not good. And as with anything mechanical it is possible to mis-diagnose especially when unfamiliar with the car.
If it's an Alfa main dealer good luck to you, these have a very poor reputation in general.
I know these comments aren't really helpful and I hope your garage can sort the fault out for you with little or no extra cost to you. Maybe someone on here has had a similar problem and can help.
This is part of the reason I went for the Qtronic due to the Alfa manual boxes having so many known issues of unreliability.