way to charge flat fee for up to 3 items.. and additional for more?

[carpenoctem]

basically i want people to pay say 100 bucks, and they can get up to 3 items (if they order 1 or 2 it is still 100 bucks).

 

anything above 3 i would like to add 50 bucks for each additional.

 

is there a way to do this??