How does Mule shipping integration work for international orders?

We are looking at ways to make international shipping more economical for our customers and ourselves. Have heard about Mule (we are UK based) where you can ship to them in a ‘Mule Bag’ which gets opened and they then send on using bulk shipping rates. Apparently it plugs into Shopify but I would like to hear from any users about exactly how etc before I go ahead. Their own site isn’t answering all my questions.

Has anyone here had experience of integrating with Mule?