I really enjoyed the example of printing multiple.

One issue I could see is if you wanted to do a single but already have invoices that are set to print but you only wanted the one.  Maybe not ready to do the all invoices just yet.  I need to play with this more.

I'd also add my vote into the hat for additional coverage on arrays.  I'm lacking in that department for sure.  
When and why they would be used as well as alternatives.