There are 10 Postcards in Traverse Town. Collect them all and toss them in the mailbox in District 1 to earn items. Here's where they are:
1. Climb to the roof of the Accessory Shop.
2. Hit the fan in the Item Shop.
3. Blue Trinity near the Item Shop in District 1.
4. Crack open the blue safe in District 1 after finishing the area the first time.
5. Jump up to the awning of the shoe store in District 2.
6. After locking the Traverse Town Keyhole, look at the paper on the wall in the Item Workshop.
7/8. Cast Thunder on the wires in District 3, then step on the three buttons in the Gizmo Shop to set it in motion. Examine the clock on the wall to get two cards.
9. Also after locking the Keyhole, climb to the top of the Gizmo Shop, follow the path from roof to roof, and take the secret entrance to District 3. There's a postcard in the corner there.
10. After clearing Monstro, Geppetto sets up shop in District 1. Go look in a pot on his shelf for the last card.
The rewards from card to card go like this:
2. Mythril Shard
9. Defense Up
10. Power Up