i can see how its frustrating... but as others have said the mechanics need to do their job and satisfy themselves that this is indeed the fault and what parts are needed for the repair. parts are not pre-ordered these days.
I went through the same thing when i had a dsg replaced last year. took around 3 months for it to be eventually changed but there is a process to go through before that will happen so i had to put up with it and return the car to them every 3 weeks or so for another diagnosis and possible solution. pita but what can you do.
if you want i guess you can supply the part when you drop off the car and let them know its there to be used if its the part they need.
good luck.