PHP in a javascript function with DB calls.

Hi All, I HOPE this is the right place for this.. if not I am sorry!!! I'm hoping someone can put me on a clear path of how to do what I am asking here. I Have a shopping cart... and in this shopping cart I want to feature some items in a pop_up window when the user is ready to check out - if they are not already selected. For example.. Someone buy a Hockey Stick but don't buy hockey tape.. when when they go to check out.. I want a pop_up to be launched asking the user if they want to add Hockey tape to their order for onlu $$$. For this to happen I need to have an onLoad on the checkout page that calls a javascript function that has PHP code to find out IF a paritcular product is selected to see if the other has been selected too, if it's NOT selected then launch the pop_up. Does ANYONE have any idea's as to how this can be accomplished?? Anyhelp is appreciated!!!! thanks all. Aaron