They've probably set the idle speed or mixture or both too low - if its a newer s3 the idle speed should be 800rpm (I do set mine a bit lower though). Much lower than that and the combined effect of lifting off quickly and slowing down causing the fuel to slop about in the float bowl can cause the engine to cut. Also, garages have a habit of setting the idle mixture too weak it should be slightly rich (to reduce NOx emissions at the expense of slight increase in CO - better for everyone's health when we're all stuck in a traffic jam!).
Setting up the carb yourself is probably worthwhile. A gas tester can help but doing it by "ear" usually gets you close enough - there are plenty of guides around on the net. A multimeter with RPM and dwell and a timing gun are good investments.