The `add_address` method is a little incongruous with the other `add_*` methods.  In all the other cases, the `add_*` methods "add to" the `post` hash that is passed in, building it up as it goes.  The `add_address` method, on the other hand, returns a new hash which the caller must merge on to the correct place in the `post` hash.

I think it would be nice if the `add_address` method behaved like the others (adding to the passed in hash), or was renamed to something else, like `translated_address_hash(options[:billing_address])`.