Jade, A possible way of putting this is as follows:
Declining request for price reduction
Gentlemen,
Thankyou for your email dated 15 September (2013?). We have carefully considered your counter proposal to our offer of woollen underwear. We regret to inform you that the requested reduction in our price cannot be met. The price we quoted in our letter of (September 12) [or] (12 September) leaves us with a small profit margin. We are sure that you will find that our prices are quite competitive with our competition.