It takes a bit of effort to set up the UPS label printing onto a UPS thermal printer. Consider the handling of 1 or more packages to domestic or international addresses. They require validation of a series of test cases before they will allow it. UPS is, by far, the easy case with a published, clear XML API. I have thought about doing it, maybe soon. Does anyone have any partial or full implementations?
USPS is possible, but much more difficult. It must handle vastly different mailing protocols for its different types of services and support a wide variety of label printers. Count me out on that implementation. Others seem less well documented.
Kirk