A typical letter might run as follows. If so I have a free gift for you - and for them. Apart from anything else, it saves expensive advertising and helps us keep our costs down. For every one who becomes a customer we will give you a free gift - and we will give each of them a free gift as a thank you for trying our service. Clearly, you should 'sell' the free gift, whatever it is, and put a value on it.
Experience is the name everyone gives to their mistakes.