In the Item Shop, attack the fan on the ceiling. This will be much easier if you lock on first. The second Postcard will give you a Mythril Shard.

Go up the stairs to the right of the Accessory Shop, then go up the stairs on the left, up past what resembles a display case. In the far right corner, you should see a blue box. Examine the box to get a Postcard. The third Postcard will give you a Mega-Potion.

Go to the Gizmo Shop located in the Second District and walk to the door on the other side. To the right will be a ladder. Walk up it past the bridge and across the rooftops until you reach a vertical hole that goes to the third district. Walk to the corner near the right to find the hidden postcard. The fourth postcard will give you a Mega-Ether.

There is a Postcard on the roof of the Accessory Shop. If you look down the hall past the stairs to the right of the Accessory Shop, you will see several boxes. Push the largest of these boxes next to the side of the Accessory shop to reach the roof and the treasure chest next to the chimney for one Postcard. The fifth Postcard will give you a Mythril.

As soon as enter the second district from the first district, look over to your right. You will see a treasure chest sitting on a ledge. Hop onto the street light on the edge of the street corner. That will get you close enough to make the jump onto the ledge. The sixth Postcard will give you an Elixir.

The clock that is stopped at "6:54" has two Postcards.

The blue trinity in the first district, NA.