@lilsiyh That’s right, there are lists with online shops for postcards. One in the German forum is kept by me. 
There are small and bigger online shops in my list, a good amount of the shops belong to Postcrossers, even if it’s not mentioned. If a user recommends a shop (not his own, that would be self-advertising), I’ll add it to the list.
Why shouldn’t they be allowed? I’m not aware that there are limitations to only big shops. Perhaps you could point out what you mean?
Concerning your suggestion, I agree with @Feuerstuhl that a limitation to small shops would be difficult. Who sets the rules which shop can be in the list? I think this would provoque discussions.