Special packaging atribute.

I have been reading on adding special packaging, but i couldnt find the answer to my dilema.
I need to add special packaging price for cooler and ice for some products out of my store. Now it usually takes one cooler to pack up different products. I was trying to add attribute, but it seams to be working by the product and anytime sometimes chose 2 different products that requires cooler they are getting charge twice for it. I just need to add attribute that will only show up once in shopping cart once the customer is ready to pay. Is this the right way to do it, or should i do something else. I have few categories with products that will require cooler in order to be shipped out.

Thank you for your response.

Konrad