So, you actually ordered it, and they did not inform you of the missing machine head before they cancelled the order?
That's messed up.
Since you had already ordered it, you should have been given first right of refusal. They should have contacted you to inform you of the missing machine head to see if you still wanted the bass, and discussed a new price. Then, if you decided you didn't want it, they could cancel and put it on clearance.
I don't like that they just cancelled your order without informing you.