Beyond the technical issues, have you considered some more practical issues? For example, to rate a package, you need to know the size and weight of the package as it will ship. Unless you can just take the product, and slap a label on it, that means you have to consider the box you will pack it in, the weight of that box, the weight of any void fill, and the dimentions. Perhaps not that hard if you're just putting one product in each shipping box, or if you have just one product that may ship in multiple units, but how will you determine shipping boxes with multiple mixed products?
And don't forget that boxes and shipping supplies aren't free when you're setting your costs.
While eBay sellers may try to match shipping charges to their actual costs, the larger the retailer, the less likely they are to try to do the nearly impossible. They may want accurate time in transit information, and they may shoot for some rough corelation between agragate shipping costs, and agragate shipping charges, but they don't try to rate actual shipping costs at the time of sale.
So while you're teaching yourself how to use the technology, don't forget that ultimately you need to figure out how to apply what's technically possible in a practical world.
