More than 4 million small parcels from China have arrived at the freight airport in Liege, Belgium, every day since the beginning of the year. On the receiving end, the Belgian customs inspection team at the airport has only 80 members.

Belgium’s top customs official, Kristian Vanderwaeren, said the airport, close to the Netherlands, Germany and France, was built, in part, to cater to e-commerce platforms such as Amazon, Shein, Temu and Alibaba.

But the “explosion” in the number of small parcels...