Snail mail is about to get a bit slower.

The United States Postal Service this week announced that, starting May 1, it will implement a slower delivery standard for nearly a third of small, lightweight packages.

The new standard enables the Postal Service to add an extra day or two to deliver about 32% of its first-class package service volume. Such packages sent through the first-class service weigh less than a pound.

Businesses in the promotional products industry and beyond will have to take the new standard into account when utilizing the affected USPS service. USPS had already instituted slower standards for nearly 40% of first-class mail last fall.

Notably, USPS pointed out that 64% of first-class package service volume will not be affected by the change. Meanwhile, 4% of the volume will actually be upgraded from a three-day to a two-day service standard.

Of note: A slower standard doesn’t necessarily mean USPS will take that amount of time to deliver. “Service standards are delivery benchmarks for how long customers can expect the Postal Service to deliver different types of mail from origin to destination — point A to point B,” USPS said. “Service standards are not the same as the percentage targets or the actual measured service performance.”

The slower standard set to take effect May 1 is part of “Delivering for America,” the Postal Service’s 10-year plan aimed at achieving financial sustainability and what it describes as service excellence.

Among other things, the longer delivery time range will allow the Postal Service to rely more on its own ground transportation, which the organization described as more reliable and affordable than air transportation.

“Modifying select service standards is a key growth element and enabler of our 10-year plan,” said Postmaster General Louis DeJoy. “This action will contribute to our cost-savings efforts and improve our reliability across all product classes, including our growing package market.”