Nearly half (45%) of holiday shoppers will return unwanted items between Dec. 26-31, according to reverse-logistics firm Optoro, with by far the most (91%) of returns happening in stores. Last year, shipper United Parcel Service said it expected to deliver a record 5.8 million returns packages to retailers during the first week of January and 1.3 million returns on Jan. 5 alone, a day the company has dubbed “National Returns Day.” Total merchandise returns account for more than $351 billion in lost sales for U.S. retailers, according to a report from retail performance improvement solutions Appriss Retail emailed to Retail Dive. While returns are technically lost sales, however, the return process actually provides an opportunity for retailers to recoup that somewhat, considering that 57% of shoppers make additional purchases while they’re in those stores, according to Optoro. Unwanted merchandise present retailers with logistics problems, especially when goods are damaged or when returning goods to shelves or warehouses is too cumbersome a process. While half of returns are re-shelved to be resold, some 5 billion pounds of returned goods end up in landfills, equivalent to the trash produced by 5 million Americans, and the returns process accounts for 1.6 billion gallons of diesel fuel and 15 million metric tons of CO2. Read more at Retail Dive.