[Bitcoin-development] BIP70: Canceling Payments

[Tim Tuxworth <tim@go-taxi.biz>]

The process described in BIP70 might be ok for a simple "happy path" 
scenario, but what if things don't work so smoothly. I'm not talking 
here about technical issues, but _very common_ business scenarios such as:

e.g. Merchant cancels request before payment is sent, such as when:-
- the merchant realizes that they charged the wrong amount
- the merchant realizes that they send the payment request to the wrong 
customer
...

e.g. the Merchant or Customer decides to cancel the transaction after 
the payment request is sent because:-
- the customer decides to pay by some other mechanism like cash or 
credit/debit
- the customer doesn't have sufficient funds and decides not to purchase
- the customer changes their mind and decides not to purchase
...

It strikes me that a "Cancel Payment Request" message is required
and a "Reject Payment Request" may also be required (or maybe use the 
same message for both).

Tim Tuxworth