As part of a Massachusetts company that's been audited by the New York, I can tell you definitively that where you ship TO is the important thing. If you ship, the item changes hands at the receiving location. If it's picked up, then it's at the pick up location.
Most of our business was in Columbia county so we tried to get away with always reporting that percent in that county, but I gotta tell you, it didn't fly come audit time...



All my alternative stink (that could be the epitaph on my tombstone, too, you know). But, that is one idea, yanking the possible choices for the customer but keeping the working mechanism behind the scenes. When the DST comes in, re-enter the choices (which would take a lot less time than the alternative) and let it rip. 
