The shoulder is the "boston butt" and the "picnic" packaged together. Together they are the shoulder. my local grocer sells a bone in, minimally processed picnic (still has some skin on) that I love. Normally weigh around 10#. Great for pulled pork.
Here's a visual on a shoulder and shoulder cuts.... the top row are whole shoulders. The bottom row shows a picnic on the left and two butts on the right. Now, if you go a little lower than the picnic you will find the trotters.