I'm having an issue and after looking around I'm not sure I know what I should be looking for and I don't understand attributes well enough to know how to fix it.

Basically I have attributes set up to sell advertisements per month. Each month has a check box. When someone checks the box for may and june, the quantity does not change to two, it stays at one. So people check out, paying only for one month of advertisements when they think they bought two. This has become very troublesome!

Is there a way to make the attributes update the quantity?

And if this is covered somewhere else, please direct me there. Thanks!