Easy Invoice was built using the same technology as Easy Books, and we've been working to add the sync service into the app. It didn't make it in time for the initial version that was launched on 12 December 2013 but it's now ready and running in version 1.1.
So now you'll be able to create estimates and invoices on any device and they'll magically sync to all your others. So if you've created an estimate for a customer on your iPad and find yourself out with only your iPhone, no problem. The estimate will also appear on your phone and you can instantly create a sale from it to send to your customer.
Rather than replacing the data on the device with the latest copy, the system merges changes. It's important because it allows for the case where all your devices are offline and you make changes on more than one of them. When they do have network access again, they'll not simply replace their data with the later copy but work through each change you made and merge them with what's already there.
Version 1.1 should be available around 21 December 2013.