ok, there are 2 different conversations going on - one is buying something in the store and having it ring up "wrong" - lower than was expected. This is not the same situation as the OP. In the OP, she ordered online, it was packed at a warehouse, it could be an intentional replacement, or unintentional slip up, or a employee who is mad at the company... I'd call and let them know.
If you purchase something in the store, and it rings up under the right amount, they have a record of the transaction - their inventory will still be accurate. The cashier will not get in trouble.
If you forget to pay for something, that is a different story all together.