You will need to provide an API that can be called back by the hosted services you use to manage and delivery your services and products.
Notes
Some common endpoints to consider providing:
Transactional Email Service:
Unsubscribe
eg: a link that automatically unsubscribes them, then depending on whether it has ajax headers or not redirects them to a page saying you have been unsubscribed.
Particular service no longer required (not unsubscribing, but no longer need tips, thanks).