A letter/email to customers rejecting an order made. It states the reason for the rejection, namely, the order size is insufficient / too small for economical production.
As per our quotation, the minimum order size is [size] as it is not economical for us to produce in smaller batches. [That said, we can still produce the quantity you have requested by increasing the unit price for each item to [amount] per unit.]
regret to inform,