Have they made any of the goods?
They can only claim any extra if they have actually suffered loss which, in this case, would involve making the goods to your specification and so rendering the items useless elsewhere.
it is very unlikely that has happened so I would not worry although the sooner you tell them the better.
I would also inform everybody in writing as soon as possible as they cannot deny receipt.
Can I clarify anything for you?
Well, they probably will.
The question though is how much is due. You cannot lawfully cancel but they can only claim the sum of their loss.