Except what you state already exists. I can load the Walmart app on iOS and order a TV from Walmart, not Apple. Apple is preventing Epic from doing the same - buy in-app merchandise through Epic's app on iOS.
Can you buy any digital goods on iOS with the Walmart app?
Yes, digital downloads for Nintendo, Xbox. Digital gift cards for Xbox, Playstation, and Nintendo, including credit for Playstation Store and Nintendo eStore.