Hidden all over Traverse Town are 10 postcards. You should mail them into the mailbox near the accessory shop at the First District. Here are the locations:
Activate the blue trinity mark in front of the cafe (1st District)
Get this from a blue safe located behind the counter near the door that leads to the 2nd District. (1st District)
Get this from a chest on top of the awning of the "Boots & Shoes" shop. Use High Jump to reach it. (2nd District)
Jump on the counter in the Item Shop and attack the fan. (1st District).
Get this from a chest on the roof of the Accessory Shop. Use High Jump to reach it easily. (1st District)
Visit Geppetto's House at the First District after completing Monstro, and examine the pot at the left corner. (1st District)
Use Thunder on the cords that are sticking out at the Third District, then go into the Gizmo Shop. There, use the moving platforms and step on the 3 square buttons. After that, check the clock for 2 postcards. (2nd District)
Jump on the furnace inside the Accessory Shop and examine the sign board on the wall. (1st District)
Go into the Gizmo Shop after defeating Guard Armor. There, use the north door to get to the back of the shop. Climb the ladder and jump on the houses until you reach a hole leading to the Third District. Check the platform here to get the final card.