I love this app. I give it four stars simply because 1) We shouldn't need an app to print branded invoices in Shopify, and 2) it's hard to edit yourself unless you know coding.
That said, we've been using it since it came out. The Shopify team helped set ours up at no charge and it provides a professional, branded invoice to ship with our orders. It includes our logo, the order info, and we had them add some boilerplate text reminding customers to check often for new items. Once that text was added, it was easy for me to find the text section in the code and change the wording when I wanted to.
Recently, I wished we could easily tell how many orders a customer had placed with us, right on the invoice, because we include different freebies with different order numbers. I'd been running back to the computer to look them up, but suddenly it dawned on me to ask if it could be included in Order Printer. Voila! Within a day they had added to our template: "You have placed (x) orders with us. Thank you!" just as I'd asked. Now not only do we know how many orders the customer has placed, but we're subtly reminding them what a great, loyal customer they are, and that they're appreciated!
Until Shopify creates a built-in invoice / packing slip template that supports branding, this is THE app to use. It's free and made by Shopify, so if it happens to break because of something they change in the platform, you know it will get fixed right away. Support is great, and there's a feature that allows you to select multiple orders to print invoices for at once.