How Do I Make Collectibles? (Like postcards etc.)

The collectibles you can you can find in game are included in specific loot-lists. For example the different types of logs will refer to different loot-lists when a sim is looking for frogs and then generate the frog the sim discovers. If you don't actually override an existing collectible with your postcards, they are new items and thus won't be included in any loot-lists. If you wish to add your postcards to one of those loot-lists you will have to alter them acordingly - for example I recall someone at mts made the Voidcritter cards findable. Please note that in doing so, your mod will conflict with any other mod which alters the same type of resource.