UPS / USPS shipping algorithm

I have to be able to calculate shipping charges based on the product(s) ordered. I'm able to communicate with UPS and USPS, parse the XML and all of that. The problem I'm running into is trying to calculate a box size. Has anyone here come up with a good solution? It is no problem at all to get shipping rates for one item. Two of the same item is more difficult, but 2 or more different items, all of different sizes, is causing me some problems. To make things worse, there aren't any standard box sizes I can use to try to fit them in. I've looked at different PHP scripts for calculating UPS and USPS shipping rates, but all of them I've seen so far expect the package size to already be calculated. -- Jason Stechschulte stech@enormousplow.com http://www.enormousplow.com Immortality -- a fate worse than death. -- Edgar A. Shoaff