The dollar value of shipping isn't the only cost - 2+ days is a long time to wait and Wal-Mart is only a couple of miles away. Some things can wait and you can plan ahead to account for shipping times, but not everything can be planned for and sometimes you just want or need something tonight.
Weekends are especially bad - once Thursday afternoon passes, you're probably not going to get what you order until Tuesday or Wednesday.
Fair enough, if you live close to a Walmart (and I know many do.) Personally, I find the 2 day wait is generally a reasonable trade for being able to order from my iPhone wherever I am, saving me an hour of driving to Walmart, going in and driving back. Moving errands from time blockers to parallel processes is a win in my view.
This is exactly what I love about Prime. Need something? Take 3 minutes to go online and order it. Then forget about it. It will come in a few days and requires no mental overhead (i.e. "remember to buy X at the market next time") in the mean time.
Weekends are especially bad - once Thursday afternoon passes, you're probably not going to get what you order until Tuesday or Wednesday.