Amazon.com's Prime Day sales were lower on the first day said Momentum Commerce. The company which manages online sales for 50 brands said its Amazon sales plunged 41% on Tuesday when compared with the start of Prime Day last year.

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The figures apply across all online retailers for Amazon Prime Days. Tariffs and economic uncertainty are on the minds of shoppers as they prepare for Amazon's summer Prime Day sales. But at least one company is forecasting that sales for the online sales event will break records as budget-conscious shoppers look for deals.