The modification to the agreement was signed Monday in Moscow with the Russian Space Agency and contains provisions for crew and cargo services through 2011.
Under the addendum Russia will deliver to the ISS 15 American astronauts and 5.6 metric tons of cargo.
"The firm-fixed price extension covers crew rotations for 15 [U.S.] crew members, six in 2009, six in 2010 and three in 2011, delivery and the removal of 5.6 metric tons of cargo," the release said.
NASA earlier announced its intentions to gradually reduce U.S. shuttle flights to the ISS as problems with the spacecraft had emerged with the latest launch, involving the shuttle Discovery, taking place last December.
"With the modification, NASA is also purchasing the use of the Russian Docking Cargo Module (DCM) to carry 1.4 metric tons of NASA cargo to the space station," the release said. "By adding the module, NASA will be able to fly outfitting hardware for the Russian Multipurpose Laboratory Module on the DCM, eliminating the need to fly a cargo carrier and ballast on a shuttle flight."
Under the signed modification NASA also purchased a flight opportunity for another astronaut in 2009.
"In addition, NASA is buying a flight opportunity to the space station that will meet an obligation to international partners. The flight will allow an astronaut from our partners to spend approximately six months aboard the space station. That flight is planned for 2009," NASA said in its release.
