The business of unwanted gifts is booming. Just ask United Parcel Service.

UPS customers have been shipping more than 1 million packages a day back to retailers this month, according to the delivery giant.

UPS on Wednesday said it expects customers to return a record 1.4 million packages on Jan. 3, an 8 percent jump from a year earlier. The company calls that day "National Returns Day." Returns generally peak in January.

The boom in returns is part of the breakneck growth in online shopping. Consumers will return about $90 billion worth of goods over the holiday season, according to Optoro, a logistics technology company that specializes in returns. UPS invested in Optoro last year.

The trend is good news for both UPS and FedEx, which stand to gain from not just the surge in online shopping but handling packages that are sent back.