Glad to help.
Taking as much stress off the shoulder by getting the shoulder into the right position in the trophy position and through the hitting phase is quite important.
Unfortunately, the most players think of reaching up as straight as possible on the serve.
I think you can see how jammed/impinged the shoulder joint looks in pic 4 below when the arm is straight up, and you don't "drop the left shoulder".
But the leaning that the pros and better players do lets the shoulder move with much less stress than the more vertical position most use. The position of the shoulder joint is more open as in pic 3.
But even pic 3 is an overestimation of the position of the shoulder angle on the best servers.
The shoulder angle is actually even lower! - like in the following pic of Sam Stosur - or if you go back to my previous posts and follow the shoulder angles through the Sampras serve.
Look closely, her arm sticking almost straight out from her side, with a shoulder angle only slightly more than 90 degrees ... made possible by her "dropping her left shoulder".
Other pictures of contact that emphasize how little angle there is between the humerus (arm bone) and the line across the top of the shoulders, largely due to "dropping the left shoulder" as Jim McLellan advised: