Shipping Classes

I’m not getting something. I have one shipping Zone set up for the US. I have two shipping classes set up – one for USPS Media Mail and one for Flat Rate. Even if I assign a shipping class of Flat Rate to a product, a choice for USPS Media Mail shows up for the customer to choose. What is the purpose of assigning a shipping class to a product if it doesn’t actually assign that shipping class to the product? Is there a way to assign a specific class and ONLY that shipping class to a product? I want to set Flat Rate shipping to T-shirts and use Media Mail for CD’s. So I ONLY want the USPS rate to show for the CD products and ONLY the Flat Rate for the T-shirts.