You get the postcard from Cueball.
Cueball will not appear at the expo until you unlock a certain conversation. It is important that you unlock him ASAP if you also want to unlock the Timely Traveller achievement, and let them eat cake achievements both of which are missable:
So to unlock Cueball. The first time you appear at the expo, go over and speak to Edna and Artie; eventually you will see Trie exit the doors. Go over to Trixie and go through the dialogue with her selecting the
What are you doing here options. Once you have exhausted the conversation with her
go back to Edna.
Talk to Edna and select the option.
"I talked to Trixie", Keep an eye on the truck in the background during the conversation cutscenes. After Edna's outburst now select
You want to get Trixie Fired Immediately during the cutscene you will see Cueball appear by the truck.
Now go over to Cueball and select the dialogue options relating to Trixie, there are three in total I believe. He will eventually give you the postcard. FIf you also select the options about whats up with his teeth he will give you algae cakes allowing you to pursue another missable achievement.
To unlcok this achievement once you have the post card open your inventory, Now press the
button again to examine the item and the achievement will unlock.