Did you consider the invoice solution from FM Starting Point?
Also for a great learning tool I like John Mark Osborne's training videos. You will build a pretty complete invoicing/contact solution from scratch, and learn beginner, intermediate and advanced techniques along the way.
His website only mentions the FM 15 training, but there is a FM 12 version out there too by him:
They are not in order.
Thanks for your respons Steve. I'll check out the resources you've posted.