Apple iPhone users may be able to pay for in-store purchases via their smartphone as soon as this week. The technology giant is gearing up to launch Apple Pay, its new mobile payment service, in Apple Stores and retail partner locations.

A number of major retailers, including Macy's, McDonald's and Staples, are expected to support Apple Pay on launch. Internal Walgreens memos reveal that Apple Pay will be available at some stores by Oct. 18, according to MacRumors.

But there are some notable holdouts. Walmart has outright rejected Apple Pay in favor of another standard called CurrentC, which is expected to debut in retail locations in 2015. Apple Pay will also be missing from Target retail stores, but the big-box retailer plans to support the service through online purchases made through its app.

Electronics merchant Best Buy is current evaluating the feasibility of adopting the system, the Daily Dot reported.

In preparation for Apple Pay’s launch, the company has started preparing its retail employees and partners with new training materials that were leaked by 9to5Mac on Sunday. The materials reveal a number of details on how Apple Pay will work.

Up to eight credit cards can be added to the iPhone’s Passbook app through an iTunes account or using the built-in camera to capture credit card information and users will have access to a list of recent transactions for those individual cards. For security purposes, Apple Store receipts will no longer contain customer details, except for the last four digits of their Apple Pay device ID.

Apple is expected to launch the mobile payment service through a software update for its iOS devices.