I suspect it'll require some sort of signup and beacon placement for the drone to know where to place the package; say by placing multiple beacons in your yard / on your building roof (for larger buildings) that designate the boundaries where the objects can be placed. The beacons could also transmit the destination GPS coordinates for en-route navigation, but gps is probably not enough for the final drop. That would have to rely on a signal from the beacons themselves.
The beacons can also act as warnings that a flight is incoming (lights / sounds, etc) and be able to do some sort of sweep if anything is blocking the landing pad.
Or perhaps a 'landing tarp' that has a pattern on it that the drone computer vision can use to determine if anything is in the way (such as expect a regular grid pattern); if any of the grid is obscured then abort.