When I first thought of the idea of the printable QR code, I was actually thinking along the lines you are... as an insert to a larger landing zone. I do think you idea is better, having the address information hard coded on the landing zone and having more of an authentication code printout being added per delivery.
Regarding multiple deliveries on a day, or not knowing what day something will deliver, I don't know if such things will be much of an issue when I think of typical uses cases for such a service. I am going to assume that 30 minute deliver will come at a premium price. Perhaps they will end up offering a subscription service like they do with the current Prime accounts. In either case, I imagine that opting to have something delivered that fast would likely mean, or perhaps require, that you are there to actually receive shipment once the drone arrives. I would think if you wanted something that fast, you would already be there in order to make use of said package that fast, otherwise why not just opt for standard 1 or 2 day delivery? As far as knowing what shipment is what, I'd would think its a safe bet to make that drone deliveries would have very accurate, high resolution tracking, perhaps similar to how one can track the location and status of any commercial airliner that you know the flight number of. I don't think there would be a question what is being delivered when. Why not even use its on board camera(s) to stream a private live video of the flight as it approaches your house, assuming there is adequate mobile coverage between point A to point B. That would be fun, at least the first couple times you see it.
As for the bird shit problem, I admit you got me stuck on that one. I suppose there would have to be some sort of backup authentication mechanism in place to handle such incidents. Maybe if primary authentication cannot be made, a photo of your LZ and/or current GPS coordinates on a map are sent to your mobile device app/email for you to approve. It may have to even make an automated call so it gets your immediate attention... I dont know, that is a tough problem to crack.