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Amazon's Prime Day runs through Friday, July 11. Photo: Klaus Galiano/Bloomberg via Getty Images

U.S. online shoppers shelled out $7.9 billion across all retailers on the first day of Amazon's four-day Prime Day sale, up nearly 10% from last year, per Adobe data.

Why it matters: Prime Day is more than just a summer shopping event — it's a pulse check on U.S. consumer behavior, pricing power and retail strategy.