One other factor, some distributors will ship on behalf of the dealer (especially for brand new high-demand items). In that case, "in stock" could actually mean in stock at the distributor. It is quite possible that Zip was relying on information they were getting from their distributor that was unclear and subject to misinterpretation by Zip (leading them to believe it was ready to ship when the distributor was actually not ready to ship). I believe Zip has a single location, so when they "ship" from anywhere other than Minnesota, it is likely the distributor shipping on their behalf.
I think you're spot on. According to their website they have 4 warehouses they ship from: Minnesota, Indiana, Texas, and Florida.