I have a listing site that will allow for various upgrades to a listing. I am thinking about handling that sale of the upgrades via MarketPress. I need to be able to set a single item ex. Upgrade A. and then dynamically connect it to a post_id so when it is purchased can pull the id from the IPN and create the upgrade.
The bit I need is passing the post id to the purchase and getting it back via IPN. Is there a MarketPress filter I could use to hook into?