I need to send some data to the accounting system that my company uses. It has a public API and endpoint where I can ping it with an XML message. 

If I was just going to send Opportunities, then I could just use Outbound Message in a simple way. 

However, I need to send Opportunities and some LineItems. I need to bundle this data together. 

I'm wondering how should I handle this? Should I write custom Apex code? Or can I do this as a Flow? Is there a simple way to do this using Outbound Message?