Some Walmarts will price match their internet prices and some will not. Not sure what the actual company policy is.Like you said if the item is in stock and cheaper online you can always order for pick up and get the lower price.
Our local store used to match sale prices by other local retailers if you just brought the ad in and it was the exact same item. They no longer do that.
No matter, Walmart prices are hard to beat. I just bought a container of Acetone at Lowes for about 7.75. Was in Walmart and decided to see if they carried it. Same size can was around 4.50. I checked the cans and both were from the same chemical company even though the brand names on the cans were different.