The business letter of apology should be written as soon as the mistake is caught. This way it has a better chance of mending a relationship that may have been broken. It should contain an acknowledgement of the mistake, what you have done to prevent the mistake again (without getting too personal), an actual "we/I apologize", and the request of continued business in the future (by giving you
. another chance). It may also contain something of a more tangible quality if your company allows such as discounts on the next purchase, free shipping, or a gift.