【答案解析】The challenges that the online retailers are now faced with are: due to higher shipping costs, the increase in sales does not lead to the increase in profits. In order to boost their sales, online traders have to subsidize shipping, which further reduces their profit-earning power. Traditional bricks-and-mortar stores like Wal-Mart are also improving their own online offers. They even offer "pick-up in-store" options for people buying online but wanting to avoid shipping costs or having to stay at home to take deliveries.