Rewarding your customer by passing on a voucher

[mash]

Hi there,

I have been trying to think of away of reward a customer for passing on a voucher to a friend once the friend has order from us.

Does anyone do that?

The way I was think of doing it:

1. in the order delivery have a postcode with a Single Use Coupon code
2. Ask the customer to write their order number on the postcard
3. Give the postcard to a friend
4. Thier friend orders from the site and uses the coupon code to get the reward
5. next to the coupon box on the website is a space where they can enter their friend's order number
6. When the order comes in we notice that is has a code and we then send a code to the original person to thank him.

Not perfect, would it be possible to have another field where people could enter an order number?

How would I do that?

Thoughts?

Thanks M