LOL . . . #2, ship what ya got and send the rest when it comes. People do use these radio buttons, it's pretty evenly spread what they will do at this point. Overall, they want their stuff, so they select that or remove the items not in stock.
This is not a "standard," keep in mind out cart is custom programmed and this is just what we do and it works out pretty well. People are pleased with the open publication of stock.