Given your layout, clear directional signage can be highly effective.

At the ground floor, indicate that Flats 1 and 2 are located there. At each subsequent floor, place signs as follows:

Level 1: Arrow left to Flat 3, arrow right to Flats 4 & 5.
Level 2: Arrow left to Flat 6, arrow right to Flats 7 & 8.
Level 3: Arrow left to Flat 9, arrow right to Flats 10 & 11.
This straightforward approach will help delivery personnel navigate efficiently.