Why not send the notification data along with a regular transaction?

[ottosch]

One question I started asking myself while doing some experimentation:

When user is about to send a payment to person A, what if the wallet allowed them to send the notification data to person B, i.e. pay someone and send the masked payment code to someone else?
The benefit on fee saving is obvious. About privacy, I don't understand if it is worse, better or the same.

A possible privacy benefit would be that maybe there are now 2 interpretations to which is the change from the notification (the payment to A and the change output).

Are there are downsides in this approach?

[justusranvier]

That behavior is described in the latest specification for payment codes:

https://github.com/OpenBitcoinPrivacyProject/rfc/blob/master/obpp-05.mediawiki#notification-output

Wallets SHOULD prefer to create of notification outputs in any situation where a change output is required, if the wallet knows of the existence of any payment codes but has not yet sent a notification. The practice of preemptive notification makes assumptions about the relationship between a notification output and the transaction in which it is contained invalid, therefore wallets which practice preemptive notification MAY include the notification output with the first payment to a new transaction recipient.

[ottosch]

Sorry, I forgot to specify I was talking about BIP47 v1.