Very cool! Invoicing is a terrible (and least interesting) part of freelancing and consulting. This looks like a great MVP. I especially like that when I print it out, that there's no big "billmebob" logo at the top there.

I think it would be fair to put a self-promotional link/tagline at the very bottom of the page though.

Also, allowing me to email directly from the page would probably be nice. At the very least, just do a mailto: link with the subject and body filled in for me to just hit "send" :)

Awesome MVP, definitely keep us up to date on the iterations!