We had the same issue with Walmart pickup, and switched to Target pickup instead. Substitution/stock issues disappeared, and we found Target pickup to be more frictionless.
I really want Walmart to succeed in online delivery because I don't want Amazon to gobble everything.
Recently, I was shopping for a media entertainment cabinet. Its for our upstairs media room, so I wasn't super concerned about it being "nice furniture". I went and checked at our local Walmart and found a piece that would work. Instead of trying to load the 180lb piece into my car by myself, I thought "I'll just order online and have it delivered to my door and take it upstairs in pieces"
When it was delivered I was out and when I returned I discovered they sent me the wrong item (desk instead of media center). Went online to start the return process and the only option given was "return to your local store".
Luckily the desk was only 90lbs, but still it was a chore to load this in a car, drive to walmart, then at walmart there's no flat handtrucks or dollys, so had to wrestle into regular cart to bring to service desk.
They processed the return quickly and had the right media center in stock, but now I was back to square one: How to load a 190lb piece in my car using a standard shopping cart?
By Amazon being a 100% online store, they were forced to develop sane and customer friendly approaches to handling these types of issues. Because Walmart has never had to deal with these things, they are very lacking in handling these edge cases.
I still plan to use Walmart online, but will be very cognizant of what I'm ordering given their return process.