The number of online purchases being returned daily will rise by 80% on Wednesday, according to Royal Mail.
The first working day of the year is when most UK consumers will return unwanted and ill-fitting Christmas presents.
The most commonly returned items are clothing (72%), electrical goods (42%) and PC software or hardware (33%).
Royal Mail's research shows that consumers increasingly want "try before you buy" services.
Two in five shoppers said they would purchase more items if retailers allowed them to use such a service.
And at least 60% of online shoppers would not use a retailer again if the returns process is difficult.
Women are more likely to return an item if it is not what they expected, while men are more likely to return a non-clothing item because it is incompatible or not useful for its intended purpose, Royal Mail found.