How to implement a successful refer a friend voucher system?

[paulson1]

Hi,

We are planning to introduce a voucher system - Refer a Friend.

If the friend who they referred uses the same email id to register and  when the friend makes a purchase of more then £100 (any time in the first 6 months), we will offer a voucher of £5 to the person who referred this friend.

Could you please suggest the best way to do this?

Thanks