Since 2012 we have integrated Adobe Business Catalyst through its API with a number of external applications, such as CRMs, POS systems, accounting packages, SMS gateway and lots of other custom and inhouse solutions. If it has an API we can integrate it.
We also run applications on our servers which pull data from BC or external files, manipulate it, then push it back to BC. Some of these applications have a custom built interfaces for data manipulation and management.
Xero is a great accounting package. We use it ourselves for invoicing and keeping a track of quotes, expenses, staffing and anything else you may expect from an accounting package. But the best thing about Xero is its API. It's solid and Business Catalyst can be integrated with it well.
We've built middleware on our servers which sits between Business Catalyst and Xero and pushes customer orders, line items and customer details from one to the other seamlessly.
Since we can customise our integrations to your heart's desire, we can do a lot more than off the shelf solutions can.