I retract my conclusion from this post. As was pointed out to me, the problem is that the delivery is on behalf of the sender. When a Jew sends a package, the mailman is acting on his behalf. If the package arrives on Shabbos, then the mailman is delivering it on Shabbos on behalf of a Jew, which is prohibited. But if a non-Jew sends it, then the delivery is permissible in itself.
However, when a Jew orders something to be delivered specifically on Shabbos, then the delivery is also on behalf of the orderer and is forbidden. That might be the case here. However, the majority of orderers are not Jewish, so perhaps the deliveries are on behalf of a group that is mostly not Jewish.
So my revised conclusion is that I’m not sure whether it is permissible to open the packages of Harry Potter. Ask your rabbi.
ANOTHER POINT: I neglected to mention that you are not allowed to take a package directly from the hand of a mail carrier. Ask him to put it down. From experience, I can say that mail carriers in Brooklyn are used to this and don’t think twice about it.