How do they know the income of their shoppers? I'm very curious of the technique they use to come to this conclusion. Although I can guess of some methods they might have available.
If you swipe a card in their store, they know everything about you...
Likely by purchasing information about credit card users. They likely include income and credit score in that info along with spending habits.
They have random sampling surveys for their customers that offer gift cards and cash. Every time I check out at brick and mortar stores I'm accosted by the cashier to go online and fill out a survey on the receipt. It's how they also track if cashiers and associates are having good customer interactions.
Walmart is a big dog in the retail sector and they have a powerful marketing & research apparatus.
They're almost certainly paying for credit card, social media, and other data, and are able to correlate it.