I haven't used that specific
Walmart, but I do use the one near home, at least once a week. They have an option to allow substitutions (or not) for everything you select. So, if you need a certain brand of bread, you can allow no substitutions--if they don't have it, you won't get bread. If they do need to substitute, they go with something of equal or higher value. On my last trip, they didn't have the cheap-o multigrain bread I usually get, so they gave me Arnold 12-grain instead. When they bring the groceries out to your car, they tell you about any substitutions or omissions. Also, the bag containing the substitution has a bright sticker on it (as do any bags with fragile items, such as eggs).