Applying Early Bird Discounts

Hi Tuan,
We have 3 shows a year. Our shopping cart lets you load all three checkout. If you make full payment Before Jan 23rd, you get a $30 early bird discount per show. But if for instance they pay in full for one show only, then the discount should only be applied once. Is there a way to do that?

We have a dropdown menu called payment type, and it lets you select between full payment and deposit payment. 

Maybe somehow this can be done with a coupon?

Thanks!