Used stamps exchanges.

Wondering if it is customary to send a message to the receiver once you have sent the stamps? Otherwise I'm not sure that there is any other way on this website to tell whom you are expecting a letter from?

I agree with Emily (Feijoa_girl)!
The surprise - is very nice thing! The main idea of this project - is random swap from random country.

Unless there is clarification needed before sending, I personally prefer the surprise. It's such a wonderful feeling to discover a letter from a far away country as a complete surprise :)

I think Yes, it should be mandatory in the rules.