How do I set a value based upon other values?
If you have a small number of settings to store, you might consider using system_settings_form() to store them in Drupal's variables table. Take a look at the core modules that implement this function for more examples.

If the rates group logically, and/or need to be edited by users, you might make a postage_rates content type with fields on that for the information that changes in addition to any constants set by an admin using the first method. You can then add a entityreference field from you product to the postage_rate.