Refactor #commercial_base? and #commercial_plus? to use ivars
Extract commercial type into a private method
Extract default service into a private method
Refactor #default_service to use a hash lookup
Extract service into a private method
Extract service validity assertion into a private method
Refactor domestic rate field logic into a lookup table
Refactor international rate field logic into a lookup table
Refactor rate response handling to have less synthetic variables
Refactor commercial flag setting to use a lookup table
Refactor commercial_type to be set in #initialize

* Remove #commercial_base? and #commercial_type? which are no longer
  needed since the logic is only used in one place.