Vivi Armacost loves Temu. She uses the Chinese online marketplace to buy crafting supplies for her purse-making hobby. “You can get purse detailing and hardware for cents and pennies,” said Armacost, who is 24 and lives in New York. She says it seems like “basically everything” in her apartment comes from Temu. Donald Trump’s 10% tariff on China-made goods sold to the US, which went into effect early on Tuesday morning, might change her shopping habits. The tariff closes off a trade loophole that allowed fast-fashion companies like Temu and Shein to ship packages under $800 into the US duty...