I've had Yahoo for years and never had an issue with logging on or order confirmations, so I guess that's not it. It's very weird. The whole thing is automated. I assume you log on to Walmart using your email as the UserName. A web search for "Can't Get Walmart EMails" finds a bunch of results, as does "Not Receiving Walmart Verification Codes," but the Code issue seems to be limited to the Walmart app on phones. I don't see anything real useful by skimming the articles, but it does seem to be an issue for some number of folks. You could Search your emails for help@walmart.com to see if the emails (order confirmations) have automatically gotten moved (it's possible a Filter was set up.) At least this will let you know if they are somewhere besides your Inbox. I believe you need to Search the Trash folder as a separate search...it might not get picked up in a general search.
I delete my trash permanently at least twice a week. Confirmations are kept in the Inbox until received and no problems are found then deleted. Since the Yahoo account has been deleted at my request I no longer have access to any of that information. As for Filters I've never used any I don't get the purpose of them.