WASHINGTON — It's probably not an exaggeration to say that the banking industry relies on the post office more than any other sector of the U.S. economy.

More than half of all statements and bills sent through the mail come from banks, savings and loans, and credit unions, according to the U.S. Postal Service. And those 4 billion or so mailings per year are matched by roughly the same number of advertising offers from credit-card issuers.

That leaves banks with a big stake in the debate on Capitol Hill over how to reform the Postal Service, which has suffered a sharp drop in revenue as a result of the recession and rapid changes in technology. Congress faces pressure to act by May 15, when a moratorium on post office closings ends.

While banking trade groups have steered clear of a public stance on a particular piece of legislation — three different bills are shaping the debate — they are hoping to avoid cuts in service and rate hikes. Facility closures and the potential that the Postal Service will be allowed to offer certain financial products are also on their list of concerns.

"Financial institutions are big users of the U.S. mail system," said Ken Clayton, executive vice president for legislative affairs at the American Bankers Association. "Obviously any cost increases could potentially have a significant impact on them."

Brian Tate, vice president of banking and securities at the Financial Services Roundtable, said: "The changes that happen are going to impact us, they're going to impact regular people in the middle of the country."

It's hard to say just how much money banks would lose if Congress makes it easier to raise postage rates above the rate of inflation, as a bill sponsored by Sen. Joseph Lieberman would allow. But it would be a considerable sum.

According to the Postal Service, financial institutions mailed more than 19 billion pieces of advertising between October 2009 and September 2010, the most recent period for which data is available. (More than three-quarters of that mail was sent at standard postage rates, which are below first-class rates, and the data does not show how much of the total was sent by banks versus other financial institutions.)

In 2011, both Citigroup (C) and JP Morgan Chase (JPM) sent out more than 1 billion credit-card solicitations, according to data collected by the research firm Ipsos.

As reliant as banks remain on the Postal Service, the reverse is also true. In fiscal year 2011, revenue from the financial industry totaled $7.1 billion, or nearly 11% of the Postal Service's total commercial revenue, according to a USPS spokesman.

But the move by banks toward paperless statements, a shift to mailing statements quarterly rather than monthly, and reductions in advertising budgets have all hurt the postal system's bottom line.

The Postal Service's fiscal condition deteriorated sharply during the financial crisis, with advertising by the financial industry falling by 5% in fiscal year 2008, 28% the next year, and 10% in fiscal year 2010, according to Postal Service data.

Bank industry officials warn that rate hikes will only give banks and other industries a greater incentive to find other channels of communication, which would further undermine the postal system's viability.

"It encourages people to use the Postal Service even less," Clayton said.

Members of Congress also recognize the negative consequences of rate increases. Much of the debate on Capitol Hill has focused on finding other ways - especially workforce reductions - of plugging the Postal Service's budget gap.

Also on the table are the possibility of a five-day delivery schedule and closures of postal facilities. Cutbacks in deliveries would hurt both the ability of banks to deliver timely notices to their customers, which are often subject to regulatory requirements, as well as receiving payments from customers.

"Those service levels should be maintained as much as possible," Tate said.

Particular banks are also seeking to ensure that postal distribution facilities in their areas are not closed. The Postal Service is currently planning to close 35 processing and distribution facilities in 17 states.

Yet another area of concern for banks is the possibility that Congress will allow the Postal Service — in its quest to find new revenue streams — to expand its product offerings to include certain financial products. Postal systems in some countries do provide financial services, but the financial services lobby in the United States has fought to resist that trend here.

Under Lieberman's bill, which is the leading proposal under consideration in the Senate, the Postal Service would get greater flexibility in terms of providing non-postal services, but only if they did not create unfair competition with the private sector.

An alternative bill sponsored by Sen. Bernie Sanders goes further, allowing the Postal Service to provide non-postal services that are in the public interest, and specifically mentioning check cashing as a possibility.

Clayton voiced concern about how community banks would fare in competition with the Postal Service. But he also said that there is uncertainty about what financial products the Postal Service would like to offer if it is given more flexibility by Congress.

"It's just not entirely clear to us what the Postal Service would want to do in this area," he said.

Supporters of giving the USPS greater flexibility argue that Congress is restricting the Postal Service's ability to generate revenue while at the same time requiring it to be self-sufficient.

"You can't really have it both ways forever," said Kristina Costa, a research assistant at the Center for American Progress, which supports giving the Postal Service greater flexibility.

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