Multiple Product Discounts in MarketPress

Hi. I have GridMarket set up on my site and I'd like to allow the option to give discounts to people who purchase more than one product.

I have seen this option work on a different platform at -- the more products people put in their shopping cart, the cheaper the products become.

I'm also looking for ways to "sexy up" the basic theme, so if there are any people on this site who specialize in taking a basic theme and personalizing it at a reasonable price, please let me know.

Thank you!