Excellent plugin, VERY extendable

Whether you’re a regular user, or a seasoned developer, if you’re sending out invoices, Sprout Invoices is the WordPress plugin for you.

If you like keeping all of your tools under the roof of WordPress, then Sprout Invoices will fit-in perfectly. It integrates seamlessly, so you can use it out of the box, of style it to your liking with custom CSS, or completely custom templates.

If you can’t fix something with the provided settings or by tweaking the template, and you are a developer(or feel comfortable exploring code), there’s a pretty high chance that you can use a filter or action hook to achieve what you need.

The Pro version is also packed with even more awesome features, and the support is outstanding.