Fixed Discount With An Entry Required To Redeem

I don't think my subject line adequately explains what I need.

I have to offer a fixed discount to a school per package ordered by the parents.

Each parent has their own Govt # that they will have to enter to get the discount.

I have to track the number they use so we don't get in really big trouble with the TAXman at the end of the year.

I did create a coupon that will give them 15%off what ever they order but I cannot see a way to make they entry of the GOVTtax number mandatory. Everyone will have a different tax number.

Is there any other way to create a space for entering a number?

Suggestions??

Thanks

Go to your photo price list (Photo Products -> Price List) and create the option for them to enter in the tax id in the  Additional Order Fields / Options  section.