[they] briefly implemented a strategy where
    one in every nine vending machine bottles
    was empty ... this effectively raised the
    price to 5.625 cents.

Instead, charge $0.10 but 44% of the time you get your dime back along with the bottle. Effective price is the same 5.6 cents, but people are excited to get free bottles instead of mad and disappointed.

(This might be illegal, since, it's a lot like gambling, but both versions should be equally illegal.)