I am selling note cards. There is no price difference between cards. Same pricing structure applies to all.

What I would like to do is calculate a total based on the total quantity of cards not individual cards. 

So, a set of 3 = $15 regardless of which 3 is selected. Right now, my paypal cart only allows me to have 3 cards of the same kind.  Can I do what I want with Magento?  And, what is the pricing structure for Magento?  

Is it a stand alone program or can I integrate with my current site?  See http://www.miriweiblefineart.com/notecards to see the site.