WASHINGTON — Wine wholesalers and amateur beer makers want Congress to repeal a law that makes it illegal to ship alcohol by mail.


Giant insurers like Aegon of the Netherlands want to make sure that the United States Postal Service stays out of the insurance business. And medical supply companies like Medco oppose the post office’s plans to cut Saturday delivery, saying the move would delay medicines and could add to the cost of mail-order drugs.


As Congress begins work this week on legislation to shore up the finances of the debt-ridden post office, companies representing a cross-section of American business are spending millions of dollars lobbying lawmakers to oppose or support various proposals to keep the agency afloat.


In total, lobbying disclosure records show that companies and unions with a stake in a postal overhaul have spent nearly $300 million in the last three years as the financial condition of the post office has worsened, though it is not known how much of that was spent specifically on postal issues.


The service is the backbone of a mailing and shipping industry that employs more than 8.5 million people and supports almost $1 trillion in economic activity every year. The service itself employs 574,000 people.


Nearly every business relies on the post office to deliver packages, advertise services and send out bills. This postal supply chain supports millions of American jobs in fields as diverse as banking, agriculture, media and manufacturing.


Benjamin Y. Cooper, a lobbyist with the Williams & Jensen firm in Washington, and a coordinator for the Coalition for a 21st Century Postal Service, said the level of interest shown in post office reform is understandable, given its importance to the economy.


“It’s not a stretch to say that many businesses literally depend on the Postal Service for their livelihoods,” said Mr. Cooper, whose coalition represents some of the service’s biggest customers, including FedEx, which sometimes hands off shipments to the post office at the local level.


But the service is losing a staggering $36 million a day as customers are increasingly turning to the Internet in place of letters, print publications and monthly bills that arrive by mail. Because of a Congressional mandate, the service has to pay $5.5 billion annually into a fund for its future retirees, adding $20 billion in debt since 2007.


To trim its operating costs, the service has proposed closing 3,700 post offices, mostly in rural areas; shutting more than 250 mail processing centers; cutting Saturday delivery; expanding into new lines of business; and increasing postal rates.


But many companies are deeply concerned about the sweeping proposals and say cuts in service and rate increases could have a devastating effect on their businesses.


Greeting card companies say they are especially concerned about proposals to raise the postage rates.


“We think increasing rates would be counterproductive for the Postal Service and for customers like us,” said Rafe Morrissey, vice president for postal affairs at the Greeting Card Association, which includes Hallmark. “The price increases would cause people to turn more to electronic mail and away from physical cards.”


Mr. Morrissey said the greeting card industry is less dependent on electronic commerce than other industries are, and about 60 percent of cards are still sent through the mail.


While rate increases and delivery are the most pressing concerns for greeting card companies, as well as magazine and newspaper publishers, some businesses are worried about the post office branching out into their lines of business.


As the service looks for new ways to raise revenue, various proposals in Congress would allow it to get into financial services or insurance, as do postal offices in European countries. And that has some insurance companies worried.