United Airlines has been testing a mobile app that will allow users to earn as many as 24 miles per dollar spent on purchases made using the app.

This fall, United began offering the MileagePlus X app to a select test group of MileagePlus members, allowing them to earn miles for purchases at participating stores. Retailers in the program include Lowe’s, Gap, Sears, AMC Theaters as well as more than 50,000 merchant locations.

To make a purchase with the app, you first need to enter the total purchase amount into the app. The app then makes a digital gift card which the store’s cashier can scan to complete the transaction. The miles earned through the purchase post immediately to your MileagePlus account. The app also offers a 25 percent bonus on miles earned through the app for Chase United MileagePlus credit card holders. Visit http://insideflyerus.wpengine.com/link/?11962 to see United’s current apps. Last year, United became the first U.S. airline to offer customers the ability to scan their passports to check in for international flights via the mobile app.

Bottom line: United plans to offer the app to all members in early 2015. Since you are using a digital gift card for the purchase, you will lose the purchase protection that is typically offered through your credit card, so users will want to consider that when making high-dollar purchases. In addition, since the digital gift cards offered through the app come in standard amounts, you may have to purchase a gift card that has a higher dollar value than the item you wish to purchase, which would leave you with a balance that you will need to remember to use at another time. The app may turn out to be particularly useful for unexpected purchases when you might not ordinarily earn miles.