If a mistake has occurred the contract may be void at common law even where it is valid at law it may be voidable in equity on the grounds of the mistake, you may be interested in Webster v Cecil 1861, where the incorrect price was given.
You are aware an error has occurred with a supplier whom you expect to be permanent, I suggest you inform them of their error at the highest level possible, not expecting any thanks for your honesty and trouble, and there may also be VAT complications. They may just take by DD the correct amount and provide you with correct documentation; even if you do not notify them they will hopefully discover their error on audit.