About 75 percent of First-Class Mail will keep its current service standards, according to USPS, and 14 percent will see an improvement in delivery times. Some individuals could experience slower times; however, USPS has stated that delivery times won't exceed the current five-day service standard for First-Class Mail.

The U.S. Postal Service has initiated changes to its service standards this month, aiming to provide "more reliable service" for Americans.

The changes hitting USPS this month, which took effect April 1, are part of a 10-year plan to save the agency $36 billion.