In many countries around the world, buying picture postcards of sights or other remarkable views works as follows: You find a store that sells picture postcards (souvenir stores, tobacco stores, stationery stores, ...); there you get one or more display stands with plenty of different photos, and you choose whichever picture postcards you want and how many you buy of them.
After travelling to several parts of PR China in the course of several years, I have found this to be quite different there: You can find picture postcard vendors, but they invariably offer pre-combined picture postcard sets. Each set comprises 10-ish picture postcards, each featuring another photo, and vendors are very unwilling to exchange any part of the set (let alone deviate from their default set size).
Freely choosing a small number of picture postcards without assembling a considerable overhead from buying several full sets: Is it possible in China, and if so, how?
As an example of my personal typical picture postcard usage: Assuming 26 different photos are available (let's name them A to Z), I might buy one picture postcard with photo C, one picture postcard with photo N, and three picture postcards with photo P.