Reminds me very much of a Seth Roberts article earlier this year which actually references a PG article:
>Here’s what I want: A price per email. A service that charges people for each email they send me (e.g., $1/email). I get most of the price, the company providing the service gets a small percentage (1%?).
Even the comments seem like they're describing the exact features here:
>A great feature would be allowing people to pay more and being able to see that in the inbox. The $40,000 email gets answered first! Seth: Yes, rank them in your inbox by how much was paid. More paid, closer to the top.