If the comment does not contain any information on the delivery, or if you do not find an obvious place in front of the customer's door, then feel free to lay it on the floor in front of the door if it is not wet or dirty.
If the cleanliness of the floor is not unsatisfactory, or eg. you deliver pizza that has no extra packaging outside the box, call the customer and ask them about the "pick off". If he/she is uncertain, you can pull out one of the coolest tricks we've heard so far: ask to put a chair in front of the door and leave the food there. :)
Another possible solution, what can help you if the floor is dirty to place the food carrier on the ground, open it and stand back a few steps until the customer comes out and takes out his order.