coupon if a customer confirms an order

[caco]

Hi,
On my website, customers can order a sample of my products for a low price( let's say 15euros). I would like to offer a reduction if the customer later orders the complete product, and I would like this reduction to be equal to the price of the sample (so, 15euros).
I hope you understand what I mean, and that my English is ok.
Do you have any idea how to do that?
I have downloaded GWCoupons, but it doesn't seem to do that....

Thanks!