Relying on the ivar being set is just weird.

lib/fake_braintree/customer.rb

@@ -40,7 +40,7 @@ def delete
 
     def customer_hash
       hash = @customer_hash.dup
-      hash["id"] ||= create_id
+      hash["id"] ||= create_id(@merchant_id)
       hash["credit_cards"] = generate_credit_cards_from(hash["credit_card"])
 
       hash
@@ -54,14 +54,14 @@ def update_customer!(hash)
       customer_from_registry.merge!(hash)
     end
 
-    def customer_exists_in_registry?
-      FakeBraintree.registry.customers.key?(customer_id)
-    end
-
     def customer_from_registry
       FakeBraintree.registry.customers[customer_id]
     end
 
+    def customer_exists_in_registry?
+      FakeBraintree.registry.customers.key?(customer_id)
+    end
+
     def credit_card_is_failure?
       has_credit_card? && FakeBraintree.failure?(credit_card_hash["number"])
     end

lib/fake_braintree/helpers.rb

@@ -16,8 +16,8 @@ def md5(content)
       Digest::MD5.hexdigest(content)
     end
 
-    def create_id
-      md5("#{@merchant_id}#{Time.now.to_f}")
+    def create_id(merchant_id)
+      md5("#{merchant_id}#{Time.now.to_f}")
     end
   end
 end

lib/fake_braintree/redirect.rb

@@ -8,7 +8,7 @@ def initialize(params, merchant_id)
       hash, query = *params[:tr_data].split("|", 2)
       @transparent_data = Rack::Utils.parse_query(query)
       @merchant_id = merchant_id
-      @id = create_id
+      @id = create_id(@merchant_id)
       @params = params
     end