Build an Online Store

I need a solution for a website with 30 products. The guy who set it up used PrestaShop. The functions we need, but are not supported (or we simply don't know how): --Products should be added to the cart in increments of 25. --Difference price points for different quantities (discount for volume) --20% Shipping charges, with a $10 minimum shipping fee (orders under $50) Are those things collectively attainable with any single cart or system? Payments are currently set up through PayPal, but that part is flexible.