title: Multiple Origin Zip Codes in Magento Checkout
body: <p>I'm working on a Magento storefront for a client.  They use dropshippers, so a single zip code doesn't do us much help.  We do have it set for the most common zip code from which the client ships, so, in many cases, it's ok.</p>
  
  <p>However, in some cases, there is a different origin zip code that needs to be used.  In more rare cases, we will have multiple origin zip codes.  When there is a zip that differs from the main one, we have stored this in an attribute called 'origin zip' (creative, huh?)</p>
  
  <p>Where should I be looking to make the modifications?  We're only using the UPS shipping method, and what I'm looking to do is, before calculating shipping, to grab whatever origin zips may be in the cart (I think we've got this part), but then, depending on the results, I may need to iterate through the shipping calculation and add the values together - i.e. in the case they order one product with an origin zip code, and another product without an origin zip code, it would have to calculate the first, then the second, and then add them together.</p>
  
  <p>If someone could point us in the correct direction of which php files or classes we'll need to modify, I would greatly appreciate it.</p>