Discount for Twitter or Facebook post

Create a pricing discount for users who post a pre designed message to facebook or twitter.

Is there a third party solution?

Any help with the any of the steps generally outlined below would be gratefully received.

from payment page:

open twitter/facebook login (allow customer to login)
automatically fill post with pre designed text
let customer press post and receive authentication that they have done so
re focus browser to payment page in some way (close fb/twitter pop up)
update payment page to include discount if correct response is received

Solution

your plan violates at least 2 items in facebook policy - you are not allowed to auto fill the post field, nor you can incentivize users for using FB:

IV. Application Integration Points

1 - You must not incentivize users to use (or gate content behind the use of) Facebook social channels, or imply that an incentive is directly tied to the use of our channels.

2 - You must not pre-fill any of the fields associated with the following products, unless the user manually generated the content earlier in the workflow: Stream stories (user_message parameter for Facebook.streamPublish and FB.Connect.streamPublish, and message parameter for stream.publish), Photos (caption), Videos (description), Notes (title and content), Links (comment), and Jabber/XMPP.