variations and coupons

Bought Marketpress because I can’t get any help at WPecommerce, still trying to figure out how to setup the product I need. We sell 18 different dressings and vinegar in 6oz and 12oz sizes, selling them individually is no problem. I am trying to find a way via variations or coupons to give someone a discounted price if they order 4 6oz bottle or 3 12oz bottles of their choice.