Trevor Raekwon Seward shot Pressley several times, alleging arrest warrant. He was charged with murder, the sheriff's office said in a statement on Saturday.
Pressley was killed on September 23 while delivering mail to Andrews in South Carolina's Lowcountry, about 100 kilometers northeast of Charleston.
Investigators linked Seward to the murder after combining a fingerprint on a package found at the crime scene, according to the warrant. Seward was seen with "what was believed to be an AR-15" shortly before the shooting, and guts were found at the scene, the warrant said.
Pressley has been in the postal service for almost 22 years, said David McGinnis, inspector responsible for the Charlotte Division of the US Postal Service.
The investigation is still ongoing, the sheriff's office said.
Seward is due to appear in court on November 15, according to a court document.
He was denied bail at a weekend hearing, CNN WCSC affiliate reportedquoting Lieutenant Daryel Moyd of the Williamsburg County Sheriff.
A Williamsburg County court clerk told CNN Seward has no lawyer yet.
Pressley's funeral was on Saturday, the day Seward was arrested, the WCSC reported.
"She was the mother of the post office," Post Director Sharon Cameron said at the service, according to WCSC. "We keep her picture as you can see. And we will honor her in that post now and forever."