An excellent piece of writing - quite tough but necessary in the circumstances. Some minor suggestions - mainly for style
Dear Mr S
Your order for 1,000 mobile phones
We regret to note that you appear to have ignored our letter of 3 May just as you ignored our previous reminder.
If you are unable to pay, at least give us your reason for non-payment.
We will allow you a further 10 days to make payment, after which time we will have to charge interest on the outstanding amount of € 16,670.
Failure to settle the final installment by that date will mean that we will be forced to deal with your company on a "cash with order basis" in future and also demand that you return the 333 (it's better to state the exact number) mobile phones for which you have not yet paid.
We trust we will be receiving your remittance within the stipulated period.
- For Teachers