People, please point out what's wrong with the above, specific
high-level design, not with "e-postage" in the abstract.
Have you read the taugh whitepaper? Some of the points it raises apply
to your sketched design.
In particular, the social resistance to pay-by-unit systems applies,
and the micropayment infrastructure impossibility applies (and even
more so, because you're positing one micropayment infrastructure per
stamp vendor, so you get less economy of scale).
Furthermore, the money paid for stamps goes to the wrong place, ie, not
to the recipients. (Stamp vendors could in principle pay recipients
who get mail with their stamps. This involves creation of _another_
micropayment infrastructure per stamp vendor.)
You write that
In a recipient-driven scheme, senders still have to be able to buy
stamps, but they will be able to choose their stamp vendor, rather
than being limited to their ISP.
Except that they have to choose a vendor acceptable to the recipient.
Keeping track of which recipients demand stamps from which vendors will
be another cost which you don't seem to have mentioned. If there are a
large number of stamp vendors it rapidly becomes almsot intractable.
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