I've been using the Brother QL-1100 label printer that can print a variety of labels up to 4 inches wide, including all the standard shipping labels. I use mine with PirateShip.com for USPS labels. Just don't buy the Brother labels that are super expensive, I get my labels from a third party supplier on Amazon. The only issue is that the Brother shipping labels are slightly larger than a true 4x6 and you have to make a slight setting change whenever you print a label with the third party labels but they are SO much cheaper than the Brother labels.

Added benefit is that the Brother printer can print regular address labels and tons of other labels, not just limited to shipping labels.

The printer runs around $179 on Amazon but that's cheaper than other brands and it's more versatile with the different label cartridges.