MBTA Silver Line service between Logan International Airport and South Station will remain free through Jan. 6, the Massachusetts Port Authority said Thursday.

The bus service, which started in 2006, has been free since June to reduce traffic congestion in airport terminals and raise awareness of public transportation. The free service was originally set to end in September.

Also on Thursday, Massport said that due to the mild winter and low natural gas prices, airfield operational costs at Logan were $8.8 million below budget for the current fiscal year. As a result, the agency will credit airlines $7.9 million worth of overestimated terminal rents, baggage fees, and landing fees. The expenses are computed annually to cover operating and capital costs for the terminals, baggage screening system, and airfield.

The board also set fiscal year 2013 airline fees at 2 percent higher than last year’s budgeted rates.