With millions of mailed ballots already pouring in, early holiday shopping and mega retail events like Amazon’s Prime Day threaten to expose vulnerabilities inside the nation’s mail service, which already is dragging from skyrocketing package volumes.

Amazon’s decision to delay its signature sales event, which ends Wednesday, prompted rivals to reschedule their usual counterprogramming. That means the resulting spike in packages runs up against the Nov. 3 election, adding a new stressor on the U.S. Postal Service as it struggles to reverse service slowdowns that experts and postal workers say could ensnare ballots.

“No one has been through this before,” said Gregg Zegras, president of global e-commerce at parcel delivery service Pitney Bowes. “We’re dealing with covid, we’re dealing with huge volumes, and now, with mail-in ballots. That is certainly creating challenges for providers of all sizes.”

Postal Service reliability is seen as a pivotal issue in an election in which an estimated 198 million Americans are eligible to vote by mail. But weeks after Louis DeJoy was installed as postmaster general in mid-June, he implemented a stricter transportation schedule that banned late and extra deliveries to cut costs. Regional vice presidents and local managers were instructed to cut work hours, which led to staffing shortages even as the agency’s 630,000-member workforce was flattened by the coronavirus pandemic. Understaffing remains a problem three months later, according to postal workers, union leaders and logistics experts.

In the first half of the year, the Postal Service delivered nearly 90 percent of all first-class mail on time. That rate fell nearly 10 percentage points in the five weeks DeJoy’s directives were in place and has yet to fully recover. Federal courts in New York, Pennsylvania, Washington state and the District of Columbia have temporarily blocked those changes, after 21 states argued that the resulting service slowdowns undermined their ability to execute free and fair elections.

Postal Service spokesman David Partenheimer wrote in an email that delivering election mail, which includes ballots, ballot applications and voter information, was the agency’s “number one priority.” Packages and election mail are processed on different equipment, he noted, while emphasizing that the agency has “the capacity to flex its nationwide processing and delivery network to meet surges in volume of mail and packages.”

Amazon launched Prime Day, which usually occurs in July, in 2017 to drum up sales during the slow summer months. But the promotional event has taken on a life of its own: The commerce giant said it sold more than 175 million products last year and generated more revenue than Black Friday and Cyber Monday combined. Analysts say this year’s two-day event could be even bigger, given its proximity to the winter holidays.

Walmart, Target, Best Buy, Banana Republic, Office Depot and Office Max all responded with their own sales events, positioning the logistics industry to move close to $10 billion worth of products, based on 2019 sales estimates.

Many of those orders will end up being processed by the Postal Service, which handles “last mile” shipping for many of its competitors, including Amazon, FedEx and UPS, along with major retailers such as eBay, Etsy and Macy’s. Even though those packages are processed separately, they frequently end up in the same delivery trucks carrying election and other mail.

In a presentation to postal industry stakeholders last week, the Postal Service reported that nearly 99 percent of ballots were delivered on time, according to Angelo Anagnostopoulos, vice president of postal affairs for mail data and marketing firm GrayHair Software. The Postal Service declined to provide The Washington Post with election mail performance data.

“According to what we see in the data that we collect, we see that ballot mail is prioritized above all else,” Anagnostopoulos said in a phone interview. “The chances of that being affected are very, very small.”

But postal workers and some analysts say they remain concerned about the delivery of election mail. Online retail sales have grown rapidly during the pandemic, and analysts say they expect an even sharper spike in the coming days because of Prime Day, as well as from the deep discounts that Walmart, Target, Gap and other major retailers are rolling out to draw in customers this holiday season.

Amazon, which accounts for about 40 percent of all U.S. e-commerce sales, has been rapidly building up its own delivery network. But it still relies on the Postal Service to deliver more than 1 billion packages a year, especially in rural areas where it can be costly and time-consuming to make deliveries, according to logistics consulting firm MWPVL International. (Jeff Bezos, the founder and chief executive of Amazon, owns The Post.)

About 30 percent of its packages are delivered by the Postal Service, according to Rakuten Intelligence. Amazon delivers more than 56 percent of its own packages and relies on UPS and FedEx for the rest.

Amazon has invested heavily in trucks, vans and airplanes to deliver packages to its 100 million Prime members within two days. Each of its 338 U.S. delivery stations processes about 40,000 packages a day that otherwise would have been handled by the Postal Service, according to Marc Wulfraat, president of MWPVL International.

“Amazon has made a huge accelerated push to get the majority of deliveries under its own control,” he said. “Often it’s cheaper for Amazon to deliver its own packages in high-density areas like cities and suburbs. Everything beyond that — in rural areas — it continues to outsource to the post office.”

Kate Kudrna, a spokeswoman for Amazon, said Prime Day orders won’t interfere with election mail.

But the Postal Service already was overwhelmed by an explosion in packages because of the pandemic, a dynamic only exacerbated by DeJoy’s changes. In Toledo, it took workers more than two weeks to clear mail and package backlogs earlier in the summer, according to Martin Ramirez, president of American Postal Workers Union Local 170.

In Massachusetts and Michigan, employees are routinely working more than 60 hours a week to process and deliver mail because of relentlessly high package volumes, according to workers in each state who spoke on the condition of anonymity to avoid retribution from superiors.

“Where our packages are sorted, that’s the one office where the mantra right now pretty much is, ‘Come in whenever you want and stay until whenever you want to’ because there’s so much volume,” said Dana Coletti, president of APWU Local 230 in Manchester, N.H. “It’s unprecedented, for sure. They load up these packages and whatnot on a belt, and there just aren’t enough people to load them. There aren’t enough people to scan them. There aren’t enough people to process them. There’s just so much of it.”

A federal judge in New York ordered the Postal Service to preapprove overtime hours to process and deliver election mail between Oct. 26 and Nov. 24. The group of voters that brought the suit, alleging that the mail slowdowns violated their voting rights, asked the judge, Victor Marrero of the Southern District of New York, to force the agency to preapprove all overtime regardless of purpose. They claimed residual delays in other postal products could spill over and harm election mail performance.

But Marrero sided with the Postal Service in denying that request, something postal workers and some experts say could leave the agency in a tenuous position: prioritize ballots, which in many states have a firm deadline for submission, or prioritize packages, which often have guaranteed shipping windows?

“I think if there’s any likelihood of delays in Prime Day or any other surge in seasonal packages, the packages might be delayed delivery [rather than election mail] because they don’t get to the truck on time,” said John McHugh, chairman of the Package Coalition advocacy group and a former secretary of the Army.

The agency has a vested interest in either approach. Ballots are profitable because election offices will cover the cost of postage if voters mail them without stamps. But packages already are reshaping the Postal Service’s business, so the agency has to guard its market share from private-sector competitors. Amazon earlier in the pandemic even threatened to drop the agency over its uncertain financial picture, according to internal documents.

Postal workers and union officials said they’ve yet to receive meaningful guidance on how to handle those conflicts. The Postal Service is haggling over six court orders on operational and staffing obligations to handle election mail. In the case before Marrero, Jones v. U.S. Postal Service, the sides agreed Saturday on language for a “supplemental guidance memorandum” with instructions for supervisors.

Postal workers say it generally takes as long as a month for orders from the very top of the agency to trickle down to the rank and file. With the election less than three weeks away, the Postal Service doesn’t have that kind of time, especially as the mailing industry enters peak season with back-to-school and holiday catalogues, businesses reopening in some states, e-commerce purchases and bundles of political advertising.

“If the Postal Service is really doing its job as a service,” said Philip Rubio, a history professor at North Carolina A&T State University and a former postal worker, “it has the resources to do both, to get people what they typically need, as well as to deliver their ballots and pick them up and return them on time.”