Our BIP (officially proposed on March 1) has tentatively been assigned number 75. Also, the title has been changed to "Out of Band Address Exchange using Payment Protocol Encryption" to be more accurate.

We thought it would be good to take this opportunity to add some optional fields to the BIP70 paymentDetails message. The new fields are: subtractable fee (give permission to the sender to use some of the requested amount towards the transaction fee), fee per kb (the minimum fee required to be accepted as zeroconf), and replace by fee (whether or not a transaction with the RBF flag will be accepted with zeroconf). I know it doesn't make much sense for merchants to accept RBF with zeroconf, so that last one might be used more to explicitly refuse RBF transactions (and allow the automation of choosing a setting based on who you are transacting with).

I see BIP75 as a general modernization of BIP70, so I think it should be fine to include these extensions in the new BIP, even though these fields are not specific to the features we are proposing. Please take a look at the relevant section and let me know if anyone has any concerns: https://github.com/techguy613/bips/blob/master/bip-0075.mediawiki#Extending_BIP70_PaymentDetails

The BIP70 extensions page in our fork has also been updated.

Thanks!

James