Shopify's Order Printer app is a free, easy to use app that allows you to print custom invoices, labels, receipts, packing slips, and more. Works best when used with Shopify Shipping.
This app requires some knowledge of HTML to fully take advantage of its customization ability. The Shopify Design Gurus can assist with minor customizations, but larger customizations will need to be done by an experienced third party designer.
View the Official Documentation