U.K. shoppers who habitually return goods bought online are on course to account for almost a quarter of the £27 billion ($35 billion) forecast returns in 2024, according to the report by the return logistics company ZigZag and the research company Retail Economics.

The Guardian reports that so-called serial returners, who make up 11% of shoppers, will send back an average of £1,400 of products each this year, totaling £6.6bn, the report found.

More than a fifth of non-food purchases made online in the U.K. are now returned to the retailer.

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The report found generational divides in behavior, with younger shoppers more likely to over-order with the intention of returning a high proportion of the items. More than two-thirds (69%) of gen Z consumers said they had bought online in this way, compared with about a sixth (16%) of over-60s.

The rise of serial returners was “likely to cause alarm for retailers,” said Al Gerrie, chief executive of ZigZag, which manages returns for retail companies including Selfridges, Boden and New Look.