How do you know the shipping is off? Please give an example page where we can reproduce the problem.
I'm surprised it works without a shipper number at all. You should be able to find one by signing up for the api service and/or contacting UPS support.
How many dollars off? What weight are you using? What is the declared box size?
You could add this code to the page after the shipping is calculated to find out more where the calculation comes from: <?php var_dump($_SESSION); ?>
Then view the source code to see the returned result because it will likely contain xml that will be ignored by the browser so you won't see everything if you don't view the page source.
Did this help? Tips are appreciated...
