The fuck they don't. Anything > $35 is free shipping. I don't have Walmart+. It says it is 1 day delivery but if I placed this order, it would likely show up in about an hour.
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It's so odd it isn't a thing where you live. Walmart is fairly universal in policy in the states I've been in/traveled through.
But ya, even though it says Fedex/UPS ( notice no USPS like Amazon ) if it is a stock item they just pass up that cost and send it to the local delivery people instead. Unlike Petsmart and other's "free" same day shipping, no way to tip the driver where as others cram it on you.
All that said, it is almost impossible to beat Petsmart on prices since you get 35% off all new autoships and you just simply remove it after ordering and get 35% again. Even with their marked up prices it is still 20% under Walmart/Amazon 99% of the time and although Petsmart needs $50 for free shipping and says 3-7 business days, it comes FedEX and usually overnighted.
Amazon had their own delivery trucks and people for about 6 months last year and it was a disaster of joggers. Quietly went back to USPS months ago.