When you send a Paypal payment via Paypal.com, it lets you add an optional message to the recipient.
Is it possible to include a message with the API? It does not appear to be in the docs:
https://developer.paypal.com/webapps/developer/docs/classic/api/adaptive-payments/Pay_API_Operation/
You can, in fact, put a memo field into the 'Pay' API operation (as well as the 'PayRequest' API operation). It is not clearly documented. It seems as though they neglected the memo field in the official docs, but on this PayPal developer page (same link as in your question) you can see that they included a memo field in several of the cURL examples towards the bottom of the page.
It was hard to find, and I came here when i Googled for help. But sure enough, I tried it myself and it works as expected. The memo is included in the notification that is sent to the recipient. That is likely what you and future Googlers are looking for.
It's called memo. PayRequest fields -
memo
xs:string
(Optional) A note associated with the payment (text, not HTML).
Maximum length: 1000 characters, including newline characters
https://cms.paypal.com/mx/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_api_APPayAPI
