Welcome to the week the United States Postal Service defaults on a major obligation. D-Day was Wednesday, when the Postal Service was 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.

[The Postal Service skipped the Wednesday payment and said at the time that it would do the same next month.]

On the one hand, there is no doubt that part of the reason 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.

It is a little startling when you first hear about the prefunding requirement. It seems to make no sense, and, as many have noted, it is something that is demanded of no other company or government agency. So why does it exist? It turns out to be one of those things that only Congress could cook up.

Since the 1970s, the Postal Service has been self-sufficient, generating money by selling stamps and offering services — and not dependent on the taxpayer. It is thus considered ’’off budget.’’ Yet part of its operations — including its health and retiree benefits — have continued to be part of the federal budget, and thus count against the federal deficit.

In 2002, it was discovered that the Postal Service was wildly overpaying its retirement obligations to the tune of $71 billion. Not surprisingly, it soon began advocating for ways to use some of that excess. One bill passed that did almost nothing to solve the problem. Later bills that would have fixed the problem, however, all ran into the same stumbling block: They would have ostensibly added to the deficit. And the Bush administration was adamant that it would veto any bill that wasn’t deficit-neutral.

Thus it was that a new fund was established in 2006 — for the prepayment of health benefits for future retirees, with the Postal Service agreeing to pay between $5.5 billion and $5.8 billion annually. The money simply goes into an escrow account, where it is invested in special issue Treasury securities. Thus does it somehow magically help with the deficit. Also, of course, no sooner did the bill become law than first class mail began to fall off the cliff. The prefunding requirement became a noose around the Postal Service’s neck.

Incapable of simply letting the Postal Service go free — imagine what that would do to the deficit! — Congress continues to micromanage it, offering various ways for it to cut costs and raise revenue. The Postal Service, for instance, wants to cut Saturday delivery to save money; a Senate bill passed in April defers that decision for two years. But at least the Senate bill offers some relief from the absurd prefunding of health benefits. It would also return some of the excess retirement funding.

The postal reform bill that has emerged from the Republican-led House of Representatives, however, does no such thing. Rep. Darrell Issa, the chairman of the committee that oversees the Postal Service, talks fiercely about the need to lower labor costs, while describing the Senate bill as a “bailout.” What he is doing, of course, is using the fact that the Postal Service is going broke to impose a slash-and-burn approach — while ignoring the central reason the post office is running out of money: Congress itself. Meanwhile, the bill that emerged from Issa’s committee has never been brought to a vote on the House floor. Nor will there be a vote anytime soon. After all, the Congressional recess starts next week.

The post office insists that the default will not affect its ability to deliver the mail. Maybe not now. But several postal experts told me that at the rate things are going, it will be out of money sometime next year. Maybe then Congress will start taking seriously the crisis it created.