Police are unclear of the motive behind the attacks
Texas has been hit by its fifth explosion this month as police hunt a suspected serial bomber.
A FedEx worker was injured after a package containing nails and shrapnel blew up at a distribution centre in Schertz, San Antonio Fire Department said.
The series of blasts which have targeted the US state in March has so far killed two people.
On Sunday, two cyclists in Austin were injured in an explosion involving a tripwire device.
The FedEx employee is believed to have suffered non-life threatening injuries in the latest incident, which happened at about midnight local time.
The FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives are investigating.
Federal agents say the package is likely to be linked to attacks by a suspected serial bomber.
The string of bombings in Austin over the past two weeks have killed two men so fare – Anthony Stephan House, 39, and Draylen Mason, 17. Four others have also been injured.
The first three devices were parcel bombs dropped off at night in front of homes in the Texas capital.
The fourth incident involving a tripwire device went off in an Austin neighbourhood on Sunday. Two men, reportedly in their 20s, were taken to hospital.
Police have said all four devices were similar and more than 500 FBI agents are helping in the search for the culprit.
Before the latest explosion, police said the bombings were being looked at as possible hate crimes and that the attacker appeared to be trying to send a message.
Austin police chief Brian Manley said: "We are not going to understand that (message) until the suspect or suspects reach out to us to talk to us about what that message was."
The National Association for the Advancement of Coloured People has described the bombs as domestic terrorism.