Shipping not calculating properly

Hello,
I have questions regarding a shipping method problem used at my site:

chris.apollohosting.com/sushisite4

When I add to cart an item, say the first shirt on the page here:

tshirts.php

the shirt is added to my cart: (cart.php).

1. The shipping appears to be almost twice the rate for normal priority mail shipping on the USPS website, when I use their calculate postage for domestic mailing of a 7 oz package.

2. And when I clear the cart, and return to say this page:

index.php the shipping charge still appears in the mini menu in the upper right hand corner of the page. It won't clear out. Yet the cart is empty.

3. The shipping rate needs total up higher when I put multiple items in the cart. For example, I put 30 tshirts in the cart to test it, yet the same 9 dollar charge was indicated.

4. Also, why do the words GRAND TOTAL appear on on the cart.php when you are shopping prior to checkout out. Should it just be a running TOTAL since people are still shopping?

5. If someone clicks on Checkout in the shopping cart page, yet does not fill in any info, shouldn't there be an Error page or something that pops up? I have noticed just a Blank page comes up.

Thank you very much for your help.

Chris