I used oscommerce before moving over to zen-cart. One of the contributions that I had really like was the UPS tracking with XML because it did not require customers to leave my site to get ups tracking information.

Here is a link to the oscommerce contribution if anyone has the knowledge to try to work it out for use in zen-cart

http://www.oscommerce.com/community/contributions,113/category,all/search,ups+xml

I have tried to figure it out myself but I will admit that my php coding knowledge is limited and nearing none beyond comparing code and making small changes.