Similar experience, I have tried early morning and late evening, while trying different Walmart's, whichever route you choose will be a bust. With substitutions at one WalMart the girl removed the items from my invoice and let me keep them, at another WalMart the substitution from the email was wrong, I told the guy I don't want that crap, he told me he would have to go inside to remove from the invoice, I told him no worries, I don't want you to go through all of that.
The shameful thing is the items that are "not in stock" or "no substitution available", if you look carefully at other shopper's buggies they have those items you are looking for. So like you, I have to end up going inside. The same tale can be told with Kroger also, I have tried them also. Moral of story if you need meat, some dairy items, frozen (depending on the criteria) or paper products you will more than likely need to go inside, other than that pick up is nice.