Welcome to the week the United States Postal Service defaults on a major obligation. D-Day is Wednesday, Aug. 1, when the Postal Service is obligated, by statute, to make a $5.5 billion payment, money that is supposed to be put aside to “prefund” health benefits for future retirees. But, with less than $1 billion in the bank, the Postal Service announced on Monday that it would not be making the payment. It has a second payment, for $5.6 billion, due in September. Unless lightning strikes, it won’t be making that one either.

On the one hand, there is no doubt that part of the reason the post office is struggling is that its world has changed mightily. Everyone knows the story: the rise of e-mail, online bill paying, and so on, have cut deeply into Americans’ use of first class mail, which peaked in 2006. Last year, the Postal Service reported losses of more than $5 billion — even though Congress allowed it to defer its annual prefunding of retiree health benefits. With or without the prefunding, the post office was eventually headed toward a crisis.

On the other hand, that prefunding requirement is an absolute killer. It has cost the post office more than $20 billion since 2007 — a period during which its total losses amounted to $25.3 billion. Without that requirement, the post office would still likely be struggling, but it would have a lot more wiggle room — and a lot more cash. (Its pension obligations are also overfunded by around $11 billion.) Not since the debt crisis has there been such an avoidable fiscal mess.