Zen Cart Version: 1.3.8

I have a Zen Cart site that has a category called "T-Shirts" and all t-shirts in the category are $20.00 each. The client wants me to set up a discount where if a customer buys 2 or more t-shirts (mix or match) the t-shirt price is reduced to $15.00 each.

So I set up a 25% off discount for 2 or more t-shirts but it is a mix or match situation. So the customer doesn't necessarily have to buy 2 of the exact same t-shirt to get the discount.

Therefore, how do I set up a discount that is at the Category level, not the Item level? So basically, I need a 25% discount applied to any 2+ items purchased in the T-Shirt category.

Please help!