Two-thirds of shoppers are predicted to return unwanted items this weekend, with the Post Office preparing for "send-back Saturday".
Research found that more than two-fifths of shoppers buy more goods online than they would in a shop.
Around 3,000 Post Office branches will be open on Sunday and hundreds of extra staff will be on duty to deal with the expected high number of returns.
Pete Markey, the Post Office's chief marketing officer, said: "We are expecting our busiest Christmas season ever for online shopping returns."