I'd be wary of including a "coupon code" that identifies myself as a potential "software pirate" when purchasing something using my name and visa card online, even if the seller has the best intentions.

Coupon codes get shared anyway and that should be taken into account when sharing them out and pricing them. I always check retailmenot.com before making purchases at web stores with a coupon code field visible in their order form.

You could always generate it server side and lock it to the ip that the original request came from.