[Bitcoin-development] Payment protocol and reliable Payment messages

[Andreas Schildbach]

On 01/14/2014 11:45 AM, Mike Hearn wrote:
>     Imagine you get a good offer (payment request) from a merchant. You
>     would like to accept that offer, however the merchant has changed his
>     mind.
> 
> 
> Usually if the merchant has not delivered, then at that point it's not a
> problem and he is allowed to change his mind. It's only if they change
> their mind *after* you pay that it's a problem, right?

It was my understanding of the spec that a payment request is legally
binding one side for the specified amount of time.

Basically I offer you to sell you these 10 eggs (described in the memo)
for this amount if you accept until this time. I have even signed this
so you can know its me who made you the offer. If you accept (by
paying), the contract is valid and there should be nothing I can do
about it (except for extreme cases which are covered by law already).

Actually what good is the payment request if its not binding?

Why do we have an expiry time in the message?