Shipping calculating wrong in checkout

We have been taking orders on-line now for a while and today someone was able to checkout for $5.50 shipping when by their order total it should have been $8.50. I tried to reproduce it, and the only way I could get the shipping to calculate wrong like that was to use two browser windows/tabs. I would add products to the cart, then proceed through checkout to the confirmation page and wait. In the other window/tab I would go to the cart and change my quantity to 5 or 5000 to increase my order total (we use banded rates). Then confirm on the first browser window. Voila! Cheap shipping on my $5000 + order.
I did test this in a virgin install of zen 1.3.8a, with the test data. I could get it to charge per item shipping charge of $2.50. Without the dual windows I would get charged $25.00 for shipping.

Anyone else have this issue? Anyone know of a quick fix or where to start?

Thanks, Matt
Can't wait for 2.0!:clap:

This problem also works if I were to add items to my shopping cart, login, choose shipping, and then 'change my mind' and use my browser back button to go back to the shopping_cart. I then update quantity, and then use forward to get back to the payment page. Hit continue, and confirm.
Interesting to note, it does work both ways--it just charges the initial quoted shipping amount.

Anybody know how to figure out if someone changed their cart, then redirect them to the shipping page?

Matt