A major trash-hauling firm has admitted its involvement in a $3.6-million Orange County price-fixing scheme and agreed to cooperate in a widening grand jury investigation of bid-rigging in Southern California, federal officials said. Officers of GSX Corp. reportedly signed an agreement to plead guilty to violating the Sherman Act by conspiring with other, unnamed firms to get commercial and industrial trash-hauling contracts. They also agreed to pay a $500,000 fine, federal officials said. Three years ago, a GSX employee was charged with laundering political contributions totaling $1,500 to two Orange County supervisors. Though federal officials refused to disclose details, a government source said the allegations include “blitzing” competitors by undercutting prices and agreeing to quote prices 20% higher than normal to fool customers into thinking artificially high prices were a good deal.