GUANGZHOU, China—As China’s economic growth weakens faster than expected and uncertainty from the trade fight with the U.S. casts a shadow over business, a surprising driver has been exports. But that may soon peter out.

American clients of Ningbo Frank Electric Co., a Chinese manufacturer of kettles and other household appliances, have been placing orders several months in advance, salesman John Zheng said. “It all looks so good at the moment,” he said. “But it won’t continue.”