Whether that's a net subsidy or anti-subsidy is an interesting question. They have both a monopoly and a fairly stringent mandate: they are required to deliver first-class mail to every mailing address in the United States (including very remote areas), six days a week, at prices that they can't set. The estimates I've seen are that the first-class mail mandate is a net money-loser, with pre-sorted bulk mail and premium services like Priority Mail the more successful products. So the USPS might actually be financially better off without this "subsidy". But Alaska and Montana senators would have a fit if rural communities lost mail service.