Amazon is expanding its same-day delivery program to more American cities, and for good reason.

The e-commerce giant has seen a lift in overall sales from cities where same-day delivery has been in place for some time, and other retailers have tried to follow suit with similar efforts to boost their revenues.

Same-day deliveries cost $5.99 for Prime members. For those without Prime, same-day costs $9.98 for the first item and an extra 99 cents for each additional item. Orders must be placed by noon, and same-day delivery is offered every day, including weekends. Not everything is included in same-day delivery, but most small popular items are available, with the e-retailer claiming over a million items can be purchased through the program.

Same-day delivery has been an important experiment for many retailers eager to take back market share ceded to Amazon. EBay offered same-day delivery late last year with its eBay Now service, and Walmart has offered same-day grocery delivery in select cities since 2011. Amazon has also offered same-day grocery delivery for some time through its AmazonFresh program.

However, same-day delivery is not easy, and rumors suggest eBay Now may be in danger of shutting down. But that hasn’t stopped Google from trying to disrupt Amazon’s clout in the space. The search giant recently teamed up with Barnes & Noble to offer a same-day delivery service for books, and other big box retailers such as Home Depot are rumored to be working on a same-day delivery service as well.