The good news about the United States Postal System is that the nature of it's default is only a semi-default.
That is because in effect, the USPS missed a $5 billion payment to another branch of the government, if there is a very serious general warning in this Peter and Paul exercise.
The general warning is that it is an all to common problem in the United States for governments to have made health and retirement benefit promises that are unlikely to be met in full, absent economic policy reforms that accelerate economic growth, one of which might well, indeed almost certainly would be, general entitlement reform.
State and city governments face rather large funding shortfalls in their retirement and health promises, just as the Federal Government does as well in Social Security, Medicare, and Medicaid, if and only if, the average rate of economic growth does not accelerates somewhat over time.
The good news is that because of the mathematics of accounting for costs and revenues over time, it wouldn't really be that hard technically to solve the general type of dilemma seen in the Post Office's semi-default.
The bad news is that the political system has to solve such problems, which is why the Post Office ran into this problem in the first place, as the Congress again abdicated it's fundamental responsibility this year, and failed to pass a real long term budget.
You can only kick the can down the road on the more general Entitlement dilemma hinted at by the situation at the Post Office, because creditors, especially unsentimental ones overseas not likely to hold Treasury debt just out of a sense of loyalty, can do math well enough to see that the Federal Reserve's balance sheet is now very much masking the inability of the American political system to make significant long term fiscal decisions.
The only good that might come out of this, is that if Greece wasn't enough of a hint, perhaps a basic service failing would be, if that is absurd, since the problem in Entitlements isn't really rocket science.
Give people time to prepare for less benefits than promised, and make that distibutionally fair, uncap Social Security taxes gradually, and continue implementing cost-saving measures in medicine.
Hopefully the political class will see that if they fail to do that, and soon, there will be no partisan advantage to be had, but rather the total opposite given cross-national experience in such fiscal crisis events.
But if you want your canary in the coal mine, the Postal System's issues are a clear one.