Walmart (WMT - Get Report) has hit the all systems go button on its new digital payment option.

The world's largest retailer said Wednesday that Walmart Pay is now available in all 4,600 U.S. stores, giving the discounter a credible threat to more widely known payment options such as Apple  (AAPL - Get Report) Pay, Alphabet's (GOOGL - Get Report) Android Pay, and Samsung Pay.

Walmart Pay is grounded in a QR code checkout system. To use it, a shopper opens the Walmart app -- which Walmart says now boasts 20 million registered users -- during checkout and scans a code displayed at the register. An electronic receipt is then sent to the app.

Walmart has been methodical in the roll-out of Walmart Pay since introducing it for the first time at a handful of stores in its home market of Bentonville, Ark., last December. On June 30, the company announced it had expanded the payment option in 19 more states, bringing the total at the time to 36.

The company has been mum on the platform's performance thus far. Walmart said on a call with reporters Wednesday that transactions using Walmart Pay jumped by 45% last week as it introduced the option to more stores. Meanwhile, 88% of the transactions on Walmart Pay have come from repeat users.

Daniel Eckert, Senior Vice President of Services for Walmart, told TheStreet the company has not seen an increase in basket size, or how much people are spending each trip, among Walmart Pay users. Eckert said the important performance metric on Walmart Pay moving forward will be whether people visit the stores more frequently.

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