I think that's where the issue breaks down. This request is sent via e-mail reply, right?

So if Bob sends a message to you, and your service sends to Bob an reply/invoice saying he needs to pay bitcoin, Bob might then mark your service's reply/invoice message as spam.

Then later when Jen sends you a message, and your service sends to Jen a reply/invoice, Jen's e-mail service (e.g., gmail) might automatically assume your reply/invoice to Jen is spam (since many others have marked their reply/invoice messages as spam). So then Jen assumes you have ghosted her without a reply, which would be unfortunate because now you will not know about the next weekend BBQ of Jens that you would normally attend.

So it is SMTP that would need to have some type protocol change to accommodate this. And little is as resistant to change as the protocol for a core internet service.

In the end, I too would like to see an attempt along these lines (to make mass spamming expensive), but over time I find my usage of e-mail to occur less and less frequently. There are exceptions, and I use a service where I can whitelist certain senders (and trigger a notification on my mobile).

So for me, I feel the e-mail spammers are actually doing us all a favor by essentially forcing us towards methods better than e-mail for communicating.