using the shipping methods and cd's

Hi, we are selling CD’s online and have created a connection to USPS for real time postage pricing.

However, what is the best way to charge in regards to the weight or per cd…cs-cart has the option to set ( more than XXX) and a charge but what if we dont know how many cd’s are bought and it is unlimited???What do I do?? Do i have to set (more than 1 charge $2) and (more than 2 charge $3) and so on or is there an option to set that will be unlimited. Ideally we want to charge and extra $0.50 postage per cd purchased???