Love the refund idea — that's a pattern I've been thinking about. "Pay to send, refund if I reply" turns the inbox into a signal market.
Right now the sender pays and gets whitelisted permanently. But a refund-on-reply model would make it feel even more like a handshake: "I valued your time enough to put up $1. You valued my message enough to respond."
Might build that as an option. What would you set as the threshold?