The U.S. Postal Service on Thursday predicted big gains in holiday shipments in December, a jump of 20 percent over last year’s holiday season.

The increase will be driven by online shopping, the agency said.

At its annual news conference to mark the onset of the year’s busiest mailing season, postal officials forecast that 365 million packages will be shipped through the mail this holiday season. In addition, the Postal Service says it will deliver almost 18 billion cards and letters between Thanksgiving and New Year’s Eve.

That’s a higher rate of growth than is predicted by the United Parcel Service and FedEx, which have estimated jumps in holiday package shipping volume of 10 percent and 13 percent, respectively. Postal officials declined to discuss their market share vis a vis these competitors.

Officials also declined to provide estimates of how much revenue the extra package business will bring to the financially ailing agency, which is projected to lose $15 billion this fiscal year.

But holiday mail volume could be a bright spot in the Postal Service’s otherwise uncertain future. Congress has not passed legislation to save the agency from further losses as mail volume declines and payments for future retiree benefits loom.

Postmaster General Patrick R. Donahoe said he has “no indications” that a bill will pass during Congress’s coming lame-duck session. “The Senate has got a bill done, and we are waiting for the House to act,” he said. “We are cheering them on.”

The Senate legislation would allow the Postal Service to end Saturday delivery and return about $11 billion it says it overpaid to a retirement fund. A House committee approved its own bill almost a year ago, but it has not gone before the full chamber because lawmakers representing rural areas oppose moving to five-day service and other proposals to cut service.