Some first-class packages may take longer to be delivered, thanks to a new set of standards the U.S. Postal Service will adopt beginning on May 1.

Snail mail: The new standards include a four- or five-day window for timely delivery, compared with the three-day standard currently in place for most first-class packages, according to NPR.

About 30% of packages will see slower delivery times, while 4% will actually be sped up to a two-day delivery standard. First-class packages are small, lightweight parcels — like prescription medications — that weigh less than 16 ounces.

“The Postal Service is continually looking for ways to improve performance and provide customers with consistent, cost-effective and reliable service,” USPS spokesperson Kim Frum told NPR. “Modifying service standards would allow for additional transport time and increased efficiencies across the networks.”

According to the postal service, first-class mail delivery slowed through the first three months of 2022, with 86.7% of mail delivered on time — short of its goal of 95% on-time delivery. On average, mail and packages take 2.7 days to deliver.

Why so slow? The post office hopes to increase efficiency by relying less on expensive air transport and “more on its own ground transportation network,” according to NPR.

The move is unsurprising, given that the postal service previously slowed the delivery of first-class mail and periodicals last fall, as part of Postmaster General Louis DeJoy’s 10-year plan to “return the organization to financial sustainability.”

Although packages and mail may arrive slower, Frum told USA Today in October that the changes will also help the reliability of delivery service times.

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USPS generally doesn’t receive taxpayer money, according to The Washington Post, instead relying on the sale of stamps and other products. As mail volume has decreased in recent years, so has its revenue. But DeJoy hopes slowing services will save between $10 billion and $17 billion over the next decade.

According to Reuters, the postal service reported a $4.9 billion net loss in the last fiscal year, and expects further losses of as much as $1.8 billion due to inflation.

Slower service, higher costs: Slower service time is only one of the ways the post office is trying to save, it also plans on raising postage rates in July.

After raising the price for a first-class Forever stamps from 55 cents to 58 cents in August, CBS News reports that stamp prices will increase to 60 cents on July 10. In less than a year, that marks a 9% increase in the cost consumers will pay for shipping.