Amazon.com Inc.’s Prime Now and Amazon Fresh delivery services have been overwhelmed by demand, a sign that shoppers are turning to the world’s largest online retailer to avoid going to brick-and-mortar stores.

Amazon on Monday warned customers that both services would have limited availability, meaning orders are being delivered more slowly than usual. The company hasn’t reduced the number of people or trucks dedicated to either service, but it has seen a surge in demand that’s straining its delivery capacity.

Prime Now is Amazon’s quick delivery service available to Prime subscribers who pay monthly or yearly dues in exchange for delivery discounts and other perks. It offers delivery of a limited selection of products similar to what is found in a convenience store in as little as an hour. Amazon Fresh is a grocery delivery service available to Prime members and part of Amazon’s foray into the grocery business.