We are building an application where we would like to get notified if the user of our application creates an invoice using sage. Does sage have any kind of service for this?
the Sage Accounting API does not implement a Webhook feature or notification system (yet).
What you can do instead is retrieving only objects that have been updated or created since the last time you synced the data. Please have a look at https://developer.sage.com/api/accounting/guides/best_practices/#data-synch-efficiencies where we shortly describe how this works.
I hope that helps.