Sorry!
I can't understand why I should pay for messages when I can send messages for free, for example through SimpleX or Briar?
Perhaps you meant that when sending messages with stamps, the relay will have an incentive to work and not interfere with sending encrypted messages? And there might even be special relays? 

 Because relay operators need to pay for server-related expenses. SimpleX relay operators also need to pay server-related expenses. 

 You didn’t write about this on your website, you didn’t say why I should send paid messages.
I'm not outraged, I just want people to understand what incentive they might have to use your program.
 

 Not have free meals. The ecash stamp will help maintain the whole system