How to re-tension front derailleur cable
Agree with others here - cables stretch only slightly, requiring one or two clicks/half turns of whatever barrel adjuster, either typically next to the headtube, or for the rear also where the cable meets the derailleur body. More than that, implies the cable actually slipped because the nut clamping it at the derailleur wasn't tightened enough.
  These are not complex issues, and every cyclist should learn how to at least make these basic adjustments, on principle to be more self-reliant, and also to free yourself from constant, costly, inconvenient trips to the shop. That said, the shop that did all the work should definitely follow up on what appears to be related to their work. Cables can also not be seated fully in the frame points etc. so another possibility exists there.