I'm not going to pay
Why aren't you paying?
They didn't do what I needed them to do.
Refusing to pay until a job is done the way you agreed on is perfectly okay. Just make sure to talk over what you agreed on and make it clear, but polite, why you are withholding pay.
I'll pay when I get it the way I ordered.
Make sure to remind them to do it how you wanted. Show them your plans and ask them to fix the mistakes they have made. If they don't, then it is their fault and you shouldn't pay for their errors.