Shipping not calculating properly

After working on this shipping issue for several days, I experimented and when setting up each Add to Cart item, in the bindings tab, instead of putting 8 oz, I simply put .5 pounds and voila! it worked. HOWEVER, It still appears there is a glitch in my calculations when using Internet Explorer 8. Firefox calculates PERFECTLY now, so what may be causing this issue?

For example, In IE8: I select 3 shirts from this page:

tshirts.php
The shirts cost 19.95 each.

I click Add to Cart. I'm redirected to cart.php
Here, I can see the total is $59.85

I decide to continue shopping, so I select Continue Shopping. I'm redirected back to
tshirts.php

When I look up at the mini cart menu in the upper right hand part of the page, I see the total is $69.65?

Now, the charge for Priority Mail from USPS is $4.90, and the total in the mini cart is two times that amount. Can you tell me what may be going on here, and exactly how to fix this?

As I mentioned, Firefox browser calculates it all perfectly, whether I choose 2 shirts or 22 shirts!

Here are a few other areas that I still have questions about:

1. On the checkout page, where they enter their billing and shipping

information, can i simply change the word "Checkout" in the blue box at the

bottom right hand corner to "Continue"? (I don't want to harm the code

in anyway, just want the word "continue" to make the site more user friendly.

Do I just change the value=

or must i change id=, etc.? here's what i've got in there now:

<input type="submit" name="Checkout" id="Checkout" value="Continue" class="eC_FormButton" />

Is that okay?

2. Can I delete all the pages that beging with pp, for example pp_checkout_success.php, pp_checkout_failure.php, etc. pages since I'm using Paypal standard?

3. When I first uploaded this site to the client's hosting company: godaddy.com, I noticed that whenever I tried to add something to cart, and then go through the process of purchasing, it would redirect to pp_checkout_success.php

This was rather frustrating, because on my other hosting company, apollohosting.com, the site worked just fine. I called godaddy.com and they said they were using PHP 5, whereas the apollohosting.com company had it on PHP4. Is there a way to make this work with PHP5? what may be missing? My syntax validates, so this is kind of odd to me. godaddy said they could move me to a linux server. I decided to just put the site at apollohosting.com since it works there. But still would love to know why it won't work properly at godaddy.com.

4. While I was able to configure the return page
checkout_success.php at paypal, where do I set it up so that if a payment via paypal is unsuccessful, the customer recieves the checkout_failure.php page? I couldn't find a place to enter this on the paypal site, is this something that is configured on MY site?

5. Once someone is at paypal page, and about to login or pay, how do i configure if they decide at last minute to cancel or if they forgot something to go back to my site? Is there a place to put cancel, or continue shopping, on the paypal page? Or does a customer just have to click there back button?

6. You know how on some sites if a person incorrectly fills out a form field when entering say, their phone number, an error page comes up telling them something like "phone number must be in correct format 602-555-1212". Or perhaps they didn't put their state in, etc., and you get an error page that tells them what they forgot to do. How do I go about setting this up?

7. Finally, would your extension Universal Email be something that would work with this site as a way of sending thank yous, special offer emails, etc? The cart is static, so would this be something useful for me?

Well, thank you so much. If it's possible that we can do another phone and webchat, I would really appreciate it. I have two more sites to build with this client and am hoping that more of your products will be of benefit to us.

Sincerely,
Chris