shipping minimum

Hi,
I am using flat rate shipping with classes and I wonder, how can I setup minimum shipping per order.
for example:
product 1 shipping is 2$ and product 2 shipping is 10$.
I want to setup a minimum shipping cost so if someon buy product 1 and product two, the shipping cost will not be 12$ (2$+10$) but the minimum I set up – for example 50$.
THX