Uber is telling the world it is ready for restaurants and stores in three cities to start building its one-hour delivery system into their retail operations.

If you're not familiar, the product is called UberRush, and it has been available in San Francisco, New York City, and Chicago on a pilot program for some time now. Some users even see it as an option on their Uber home screen.

On Thursday, Uber announced its UberRush API is going to be available soon for any business that wants to build the service into its app or point-of-sale system. Uber already works with a lot of companies, such as Delivery.com, 1-800-Flowers, and Rent the Runway to offer customers delivery across town. 

Opening UberRush up to a larger ecosystem is a huge step. It means that Uber is confident it will soon be able to provide enough drivers and bicycle messengers in those cities to accommodate all sorts of deliveries--a fur coat, an engagement ring, a box of Kleenex--and will keep expanding to do so. 

"We're hoping that one day through the UberRush API, getting anything in your city will be more affordable and reliable than getting in your car to pick it up yourself," the company said in a press release.

I hopped on the phone this week with Calvin Lee, an Uber product manager who has been working on UberRush. He said testing on the project began two years ago with just a few retailers.

"We were very careful in terms of who we wanted to work with. They represent trusted brands that match the same global footprint Uber has," Lee said.

By last year, Uber was working with a few big retailers, and during the holiday season partnered with Shopify on a campaign for buying last-minute gifts. It offered free one-hour delivery for partners, many of whom were local brands. Today it says it works with thousands of businesses, though many that Uber works directly with, such as such as Google Express, EatStreet, and Olo, power delivery for hundreds of other smaller retailers.

"We all have those days when we just can't get out of the house and make it to the store," said Jaron Waldman, the co-founder and chief executive of Curbside, an app that does shopping for its customers at local stores. He said now his customers have more options--either "Curbside pickup or Uber delivery right to the door."

Things didn't start out quite as smoothly as that, though. Uber managed to build partnerships, including with Trade Global, which runs logistics for luxury retailer Cole Haan, when it began its testing two years ago. But it didn't have a technological infrastructure to support actually implementing UberRush. The back end wasn't sophisticated enough to handle dispatch from anything other than individual Uber accounts. 

Would it work? There was no way to know but to test it in a store. So, according to Lee, Uber sent an employee to a Cole Haan location in Manhattan. His job: summon an Uber every time a customer didn't want to carry his new shoes home.

"An employee actually sat in the store with 20 cell phones, and just would request an UberRush," he laughed. "Now we are so glad they can have a real customer experience and have it integrated into their own systems."