Chapter 4
Seaweed Gift Shoppe in the Seaweed Fossil Chasm.
Buy both the postcard and stamp.
Go to Chapter 3 - La Borgoi Pivot and enter the House of the Widow. Upstairs on the table is a pen.
Combine the three items in the inventory. Go to Lupis Square, facing the rope coming from the hospital window. On the left of the screen is a postbox. Use the postcard here. The achievement pops here, but to see the full effect...
*** Spoiler - click to reveal ***Go to Chapter 2 - Wall Street and enter the friends house at the back right. Inside his house, you see your postcard has been framed and he is grinning ear to ear!