This allows a site owner to choose which USPS shipping methods will be available
to the customer. Settings can be changed in the admin interface under
Modules->Shipping->United States Postal Service.
For example, if you select "Express" and "Priority", then a customer will only
be shown those two options for USPS shipping.
NOTE: as a side benefit, you can now have a list of configuration options for a
given configuration_key with the new tep_cfg_select_multioption() function.