diff --git a/lib/usps/request/package/base.rb b/lib/usps/request/package/base.rb
index ebb5acc..8c4d13c 100644
--- a/lib/usps/request/package/base.rb
+++ b/lib/usps/request/package/base.rb
@@ -6,7 +6,7 @@ class Base
     attr_accessor :size
     attr_accessor :width, :length, :height, :girth
 
-    @@required = [:id, :pounds, :ounces, :size]
+    @required = [:id, :pounds, :ounces, :size]
 
     def initialize(fields)
       fields.each { |name, value| send("#{name}=", value) }
@@ -19,13 +19,17 @@ def initialize(fields)
         end
       end
 
-      @@required.each do |field|
+      self.class.required_properties.each do |field|
         error "#{field} is required" unless send(field)
       end
     end
 
     protected
 
+    def self.required_properties
+      @required + (defined?(super) ? super : [])
+    end
+
     def error(message)
       raise ArgumentError.new message
     end
diff --git a/lib/usps/request/package/domestic_package.rb b/lib/usps/request/package/domestic_package.rb
index 40291b3..183820b 100644
--- a/lib/usps/request/package/domestic_package.rb
+++ b/lib/usps/request/package/domestic_package.rb
@@ -6,7 +6,7 @@ class DomesticPackage < Base
     attr_accessor :value
     attr_accessor :amount_to_collect
 
-    @@required += [:service, :origin_zip, :destination_zip]
+    @required = [:service, :origin_zip, :destination_zip]
 
     def initialize(fields = {})
       if fields[:service] == 'FIRST CLASS' and !fields[:first_class_mail_type]
diff --git a/lib/usps/request/package/international_package.rb b/lib/usps/request/package/international_package.rb
index 633394e..81105b7 100644
--- a/lib/usps/request/package/international_package.rb
+++ b/lib/usps/request/package/international_package.rb
@@ -2,7 +2,7 @@ module USPS::Request::Package
   class InternationalPackage < Base
     attr_accessor :country, :mail_type
 
-    @@required += [:country, :mail_type]
+    @required = [:country, :mail_type]
 
     def initialize(fields = {})
       super
diff --git a/spec/response/international_shipping_rates_lookup_spec.rb b/spec/response/international_shipping_rates_lookup_spec.rb
index d82eb49..bb665ec 100644
--- a/spec/response/international_shipping_rates_lookup_spec.rb
+++ b/spec/response/international_shipping_rates_lookup_spec.rb
@@ -17,7 +17,7 @@
 
       package.services[1].tap do |express|
         express.id.should          == '1'
-        express.rate.should        == '103.00'
+        express.rate.should        == '73.45'
         express.description.should =~ /Express Mail/
       end
     end