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walmart has some full sized lanes for self checkouts. i use those when available if i have more than a handul of items.If you've got 15 items you're approaching the limit of self-checkout time savings, depending on lines of course, and depending on the quality of the self-checkout technology. Walmart has great self-checkout hardware/software, usually pretty quick, Kroger's is awful, usually lots of errors. Kroger I'm probably going to a human cashier with 15 items if the lines are comparable, Walmart 15 items is no problem.
One glitch i found using self checkout paying with Walmart pay is i found out you cant use it twice in a row. I had a $25 walmart giftcard on my walmart app, so i split my order up. paid with the gift card first for the items that came close to $25 total, and whe i rang the rest of the stuff and tried to pay again usuing walmart pay, it kept giving me an error, sayng payment declined. after a few attempts, i used my debit card (the same one linked to walmart app) and it went right through.
With walmart pay and tap to pay everywhere else, i use my phone 99% of the time, but walmart doesn't use Tap topay on thier system.