For more details on this - usually labels are created in a file format called EPL [1] (or EPL2, or ZPL). This is a format specifically for label printers, and t

For more details on this - usually labels are created in a file format called EPL [1] (or EPL2, or ZPL). This is a format specifically for label printers, and the specification makes it easy ("easy") to command a printer to print, say, barcodes in almost any style without having to create some sort of graphic. 4x6" is pretty common. UPS and Fedex's APIs allow you to specify a label size as input, and they return a base64-encoded EPL file as output. You can then take that file and send it to your printer. This works fairly well with CUPS, which is how I've automated our shipping (just make sure you use `lpr -o raw` so that it treats that EPL file as raw input. Otherwise, the printer sometimes prints the label as an ASCII file.) I'm unsure about the USPS api - but I would argue that most high-volume shippers do not use the USPS, due to lower-quality detailed tracking info. When you print out the shipping label on paper and tape it to the box, you are using the courier's api's "GIF" or "PDF" feature, where it returns the label in one of those formats. This is great, but in my experience, it is pretty hard getting them to print properly on your label printer. Finally - and not a lot of people know this - UPS and Fedex will give you a label printer, and labels, for free! If you have high volume (unsure of the threshold) then you should call to negotiate your rate and have them bring you hardware (and set up your computer) for shipping. [1] http://en.wikipedia.org/wiki/Eltron_Programming_Language http://en.wikipedia.org/wiki/Eltron_Programming_Language