Amazon is expanding its same-day grocery delivery service to 1,000 cities and towns across the U.S., the company said Wednesday.

Amazon has positioned itself as America's most prominent shopping retailer, and a large part of that dominance is the two-day shipping offered for most items on the site. Customers with Amazon Prime are able to have items to their door within 48 hours of ordering,

The move places Amazon in more direct competition with Walmart+ and Instacart by lowering friction for customers to buy fresh food alongside nonfood items in a single checkout. Lower order minimums and free same-day delivery for Prime members could also nudge consumers toward Amazon and pressure rival delivery margins.