Where did you take the shoulder measurements from? Put the jacket flat against a surface and use a measuring tape to measure the width across the back of the jacket for the shoulders. For a 42, it should be between 19" and 20", maybe a little more or less depending on cut.
The two places that are the most difficult to alter are the shoulders and the jacket length. Unless you have a very, very capable tailor, you're going to pay a lot of money for a poor product.