I too want multi-vendor shipping origins. It looks to me like this wouldn't be that hard to implement for US zip codes, but before I reinvent the wheel, I want to see if anyone else has solved this yet, and/or if my plan will work (at least for my needs).
It looks to me like SHIPPING_ORIGIN_ZIP is what determines the shipping cost. That is used to calculate shipping in three places, that I've seen -- the UPS shipping module, the USPS shipping module, and google checkout (plus there must be a fedex addon where it is used, which I haven't installed).
What I am investigating doing is setting up a zip code per manufacturer. Since each of my products is related to a manufacturer, when I need the zip code, I'll go to the product's manufacturer, and get the zip code there. If there is no manufacturer, or that zip code is blank, I'll revert back to the store zip code. Or, I could just use the vendors from ty's drop ship module to set the zipcode.
The second part, which is much more complicated, is to break the shipping from a single cost into an array or table, with each unique origin getting its own cost / line. So, shipping goes from a single entry to multiple, but that is really what is needed (that's how Amazon handles it too). I'm sure this second part is more sticky, but really doesn't seem unmanageable to me.
So, looking for feedback on whether my approach will work.
Advantages: customers pay true shipping costs, so no approximations / averaging.
Disadvantages: More code/db changes / another module to install. More importantly, customers will see that all the products do not really come from a single store, but are shipped from different locations. However, they'll figure that out when they receive multiple packages anyway.
So -- feedback on this idea please. What am I missing? Is it much more complicated than I realize? Are there other areas that get touched that I haven't factored in (like the invoices and packing slips association with the drop shipper module)?
What I'm trying to do is get the real requirements up front -- maybe I won't solve this for global shipping, but if I can do just the US, that will help enough people out.
If I know everything that is needed, I'll see how bad it is to actually implement. Doesn't look intimidating yet, but maybe I just don't know better
Will update Zen Cart v1.3.8 if I do it.