Online grocery shopping has skyrocketed forAmazon.comInc. since the company purchased Whole Foods Market for $13.7 billion, Bloomberg reports.

One Click Retail, which monitors sales, estimates that within one week, online sales of the Whole Foods private label 365 brand totaled around $500,000, according to Bloomberg.

The speed at which Amazon began selling Whole Foods products online would have taken months in a brick-and-mortar chain, Spencer Millerberg, CEO of One Click Retail, told Bloomberg. Only 7% of Whole Foods top 100 best-selling items remained in stock on the website, and One Click Retail believes Amazon was unprepared for how quickly the new products would sell. The Whole Foods “seal of approval” may have helped.

Millerberg is skeptical about whether Amazon will be able to maintain their online inventory while using the stores to reduce delivery costs.

During the first two days after the Amazon purchase, Whole Foods brick-and-mortar locations saw a 25% increase in customers, according to Foursquare Lab Inc., which compared costumers’ mobile location before and after Amazon’s investment.