>> A more scalable approach would be for the user to send the name and
>> signature of their "instant provider" every time and the merchant just
>> chooses whether to ignore it or not, but as Lawrence points out, this is
>> incompatible with the provider charging extra fees for "instantness". But
>> should we care? I'm trying to imagine what the purchasing experience is like
>> with optional paid-for third party anti-double-spend protection. Ultimately
>> it's the merchant who cares about this, not me, so why would I ever pay?
> I think you are wrong here.
> Just because up to date credit cards charged the merchant which in turn
> charged you and the ordinary cash payer doesn't mean a newer and better
> system can't be transparent from day one.
I don't think a whitelist of trust is a practical approach because you are going to want to have varying levels of trust in different instant providers. This would be based on how large their past transaction volume has been. For that reason maybe another approach is an additional negotiation message between the merchant and wallet. Merchant sends payment details -> wallet responds with their instant information requesting if an instant transaction will be accepted for this transaction. Merchant weighs the risk based on historical data about this particular instant provider and the amount of the requested transaction -> Merchant responds yes or no.
That approach avoids the scaling issue, but also allows for Lawrence's use case of charging the user a fee only in the case where the instant transaction is supported.