The term 'stamp' is a metaphor. What is affixed to Keychat messages is not a specialized stamp, but rather generic Bitcoin ecash. 

Why does the traditional postal system require stamps? Why not just affix cash or paper money directly onto the envelope? It's likely because the cash could be stolen, and the glue might damage the paper money. nostr:note1g7v8x5dzy45q9jkxhug5h3sykrldl40zvpuyzfgnmsajuntxxcksgdzekc