Too often in our relentless pursuit of quiet backgrounds and running refrigerators, we sometimes forget that as much as sound mixing is a technical art, it is also a business.
Since a good portion of a freelance mixer's work will likely be non-timecard, invoices are necessary. If you're as averse to the benumbing slog of paperwork as I am, you'll appreciate a simple, intuitive invoicing program, one that offers flexible analysis and tracking as well as a humdinger of a professional looking result.
I had used ProfitTrain for the past five years or so. I needed something for OS X that would offer a personalized invoice template and overdue payment tracking at an affordable price, and it met all of those criteria. However, it is an app that runs on a desktop, and thus requires me to always use a particular computer. While my primary hardware is a laptop (and thus portable) , it's still heavier than a tablet, or even a phone. What I needed was something cloud-based, accessible from anywhere, but also low cost.
Eventually I found Invoicable. It's web based, which means I can sign in from a browser on anything with Internet access, regardless of platform. While it's more comfortable to type on a full-sized keyboard, I've been able to invoice a client from set before I'd even loaded the kit into my car, all from my phone.
You have the option to email invoices as HTML, as well as receive payments online. I haven't used these features, as I typically email a PDF to the client and wait for a check. It offers a freemium model, with an unlimited free version that only places a discreet, single-line ad for the service itself at the bottom of the invoice. I honestly didn't mind the ad, but chose to purchase the full license anyway because I want to support the company. (I am not affiliated with Invoicable in any manner, nor am I receiving any compensation for this post. I'm just impressed with the product.)