In addition to the USPS, Pirate Ship is now offering UPS as a shipper when you purchase labels. They search pricing based on the size and weight of your package and display the least expensive option as your first choice. You can of course manually switch the shipping method if you have a preference.
Visit PirateShip.com for details.
We have been recommending buying labels from PirateShip.com for a while now because their service is simple and they offer the best prices available. You can get the same prices form other vendors, but we find the user interface and process on PirateShip.com to be simple yet powerful.
We make no money from them so this not a paid promotion for their site. We just like their tool.