Using ZC 1.3.8a (very nice, love it!)

Anyone know of a way to obtain ZC shopping cart item line data via an external program, in order to then populate back into the cart that same data along with an externally calculated shipping charge?

FWIW, the only way I can find accurate shipping from a distributor to my customer is to get the item numbers out of the ZC cart, stuff them into the distributor cart and obtain the shipping fee, then re-stuff the ZC cart along with that distributor calculated shipping fee. I don't need help writing a program that can manipulate and read a website programmatically, I've done that several times before. I'm just wondering how to get the data out of the ZC cart and back into the cart (along with a now accurate shipping fee) at the moment my customer decides to check out of my store... I assume this is going to require a hack of the shopping_cart.php file? There aren't any "hooks" already existing, are there?