Percentage off for every 5th customer

[deborahgsmith]

I have a client that wants to offer a 10 percent discount to every 5th customer as kind of an incentive / see if you can win / contest type of thing.

 

I dont thnk I can key on the customer number because then they will never get the percentage off again unless they re-register or somethig ghappens to their customer details in the DB.

 

and I dont think I can key on the order number because that is assigned AFTER they have paid the money already via Auth.net.

 

Any ideas?

 

Deborah