The butt is the part of the shoulder above the elbow, and the picnic is the part below the elbow. Together, they are the shoulder.
I think the lower leg part has very rich meat which I like as a component of BBQ. I frequently cook butts and picnics in about a 2:1 ratio so I have some of the picnic meat to mix in.
Around here full shoulders are usually sold with their skin on. You can cook it as is or remove some of the skin. Either way your yield will be slightly less with a shoulder than with a butt, so factor that in when thinking about actual cost.
Costco almost always labels their boneless butts as boneless shoulders, FYI.