People in Cuba are enduring food shortages, near-constant blackouts and suffocating heat.

For many, the only relief comes in packages from relatives hundreds of miles away in Miami.

As the island nation faces economic collapse under the U.S. oil blockade, South Floridians are rushing to ship boxes stuffed with canned meats, bags of rice and beans and other staples to hungry relatives. They are also sending mosquito nets, flashlights, fans and loosefitting nightgowns for coping with insufferable nights. Some pay off-the-books couriers known as “mulas,” or mules, who fly to Cuba to deliver goods or envelopes of American cash.

Jorge Smith, 64, who left Cuba for Miami four years ago, has been shopping for a stronger solar-powered generator for his daughter and her 5-year-old son in Havana. With electricity ever more fleeting, the 60-watt machine he bought and shipped them months ago no longer suffices.

“They only have two hours grid power a day,” said Mr. Smith, an Uber driver who, like many Cuban Americans, struggles to pay his own bills in an increasingly unaffordable Miami.

While he is deeply opposed to the Cuban government, Mr. Smith doesn’t agree with the blockade. “By cutting off the oil, they cut off the life of the people,” he said. “It’s the people who suffer.”

Cubans have long relied on relatives in the United States, who today can turn to informal couriers, multiple shipping companies in Miami and Amazon-style shopping sites that arrange deliveries to the island.