diff --git a/README.md b/README.md
index 3c87f73..2ac60d8 100644
--- a/README.md
+++ b/README.md
@@ -53,7 +53,7 @@ To test Stripe payments, use the following test card details:
 
 1. Go to the [Stripe Dashboard](https://dashboard.stripe.com) and create a new webhook for your production environment.
 2. Set the endpoint URL to your production route (e.g., `https://yourdomain.com/stripe_webhooks`).
-3. Select the events you want to listen for (e.g., `checkout.session.completed`, `customer.created`).
+3. Select the events you want to listen for (e.g., `checkout.session.completed`, `customer.created`, `customer.deleted`).
 
 ## GitHub Actions, Linting and Security Auditing
 
@@ -135,6 +135,16 @@ See all options for running specs
 bin/rspec --help
 ```
 
+## Code Coverage
+
+[Coverage]: https://docs.ruby-lang.org/en/3.3/Coverage.html "API doc for Ruby's Coverage library"
+[SimpleCov]: https://github.com/simplecov-ruby/simplecov "A code coverage analysis tool for Ruby"
+
+[SimpleCov][SimpleCov] is a code coverage analysis tool for Ruby. It uses [Ruby's built-in Coverage][Coverage] library to
+gather code coverage data, but makes processing its results much easier by providing a clean API to filter, group, merge, format,
+and display those results, giving you a complete code coverage suite that can be set up with just a couple lines of code.
+SimpleCov/Coverage track covered ruby code, gathering coverage for common templating solutions like erb, slim and haml is not supported.
+
 After running your tests, open `coverage/index.html` in the browser of your choice. For example, in a Mac Terminal,
 run the following command from your application's root directory:
 
diff --git a/app/controllers/webhooks_controller.rb b/app/controllers/webhooks_controller.rb
index a635924..714dd8a 100644
--- a/app/controllers/webhooks_controller.rb
+++ b/app/controllers/webhooks_controller.rb
@@ -59,8 +59,8 @@ def stripe # rubocop:disable Metrics/AbcSize,Metrics/MethodLength,Metrics/Cyclom
       end
     when "customer.created"
       customer = event.data.object
-      user = User.find_by!(email: customer.email)
-      user.update!(stripe_customer_id: customer.id)
+      user = User.find_by(email: customer.email)
+      user&.update!(stripe_customer_id: customer.id)
     when "customer.deleted"
       customer = event.data.object
       user = User.find_by(stripe_customer_id: customer.id)
diff --git a/spec/models/order_item_spec.rb b/spec/models/order_item_spec.rb
new file mode 100644
index 0000000..535e357
--- /dev/null
+++ b/spec/models/order_item_spec.rb
@@ -0,0 +1,54 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+RSpec.describe OrderItem, type: :model do
+  describe "db_columns" do
+    it { should have_db_column(:order_id).of_type(:integer).with_options(null: false) }
+    it { should have_db_column(:product_id).of_type(:integer).with_options(null: false) }
+    it { should have_db_column(:stock_id).of_type(:integer).with_options(null: false) }
+    it { should have_db_column(:order_code).of_type(:string).with_options(null: false) }
+    it { should have_db_column(:product_name).of_type(:string).with_options(null: false) }
+    it do
+      should have_db_column(:product_price).of_type(:decimal).with_options(
+               null: false,
+               default: 0.0,
+               precision: 12,
+               scale: 2
+             )
+    end
+    it { should have_db_column(:size).of_type(:string).with_options(null: false) }
+    it { should have_db_column(:quantity).of_type(:integer).with_options(null: false) }
+    it { should have_db_column(:subtotal).of_type(:decimal).with_options(null: false, precision: 12, scale: 2) }
+  end
+
+  describe "db_indexes" do
+    it { should have_db_index(:order_code) }
+    it { should have_db_index(:order_id) }
+    it { should have_db_index(:product_id) }
+    it { should have_db_index(:stock_id) }
+  end
+
+  describe "associations" do
+    describe "belongs_to" do
+      it { should belong_to(:order).inverse_of(:order_items) }
+      it { should belong_to(:product).inverse_of(:order_items) }
+      it { should belong_to(:stock).inverse_of(:order_items) }
+    end
+  end
+
+  describe "validations" do
+    describe "presence" do
+      it { should validate_presence_of(:order_code) }
+      it { should validate_presence_of(:product_name) }
+      it { should validate_presence_of(:size) }
+      it { should validate_presence_of(:quantity) }
+      it { should validate_presence_of(:product_price) }
+    end
+
+    describe "numericality" do
+      it { should validate_numericality_of(:quantity).is_greater_than(0) }
+      it { should validate_numericality_of(:product_price).is_greater_than_or_equal_to(0) }
+    end
+  end
+end
diff --git a/spec/models/role_spec.rb b/spec/models/role_spec.rb
index 464a8ff..4c0397b 100644
--- a/spec/models/role_spec.rb
+++ b/spec/models/role_spec.rb
@@ -37,14 +37,16 @@
     describe "format" do
       subject { build :role }
 
-      it "name accepts a valid value" do
-        subject.name = "role"
-        expect(subject).to be_valid
-      end
-
-      it "name does not accept an invalid format" do
-        subject.name = "role-1"
-        expect(subject).to be_invalid
+      describe "name" do
+        it "accepts a valid value" do
+          subject.name = "role"
+          expect(subject).to be_valid
+        end
+
+        it "does not accept an invalid format" do
+          subject.name = "role-1"
+          expect(subject).to be_invalid
+        end
       end
     end
   end
diff --git a/spec/models/subscriber_spec.rb b/spec/models/subscriber_spec.rb
index 44778da..8e0e71c 100644
--- a/spec/models/subscriber_spec.rb
+++ b/spec/models/subscriber_spec.rb
@@ -31,14 +31,16 @@
     describe "format" do
       subject { build :subscriber }
 
-      it "email accepts a valid value" do
-        subject.email = "subscriber@email.com"
-        expect(subject).to be_valid
-      end
-
-      it "email does not accept an invalid format" do
-        subject.email = "subscriber@"
-        expect(subject).to be_invalid
+      describe "email" do
+        it "accepts a valid value" do
+          subject.email = "subscriber@email.com"
+          expect(subject).to be_valid
+        end
+
+        it "does not accept an invalid format" do
+          subject.email = "subscriber@"
+          expect(subject).to be_invalid
+        end
       end
     end
   end
diff --git a/spec/models/user_spec.rb b/spec/models/user_spec.rb
index 536b5f1..429704b 100644
--- a/spec/models/user_spec.rb
+++ b/spec/models/user_spec.rb
@@ -4,50 +4,47 @@
 
 RSpec.describe User, type: :model do
   describe "db_columns" do
-    it { is_expected.to have_db_column(:email).of_type(:string).with_options(null: false, default: "") }
-    it do
-      is_expected.to have_db_column(:encrypted_password).of_type(:string).with_options(null: false, default: "")
-    end
-    it { is_expected.to have_db_column(:reset_password_token).of_type(:string) }
-    it { is_expected.to have_db_column(:reset_password_sent_at).of_type(:datetime) }
-    it { is_expected.to have_db_column(:remember_created_at).of_type(:datetime) }
-    it { is_expected.to have_db_column(:sign_in_count).of_type(:integer).with_options(null: false, default: 0) }
-    it { is_expected.to have_db_column(:current_sign_in_at).of_type(:datetime) }
-    it { is_expected.to have_db_column(:last_sign_in_at).of_type(:datetime) }
-    it { is_expected.to have_db_column(:current_sign_in_ip).of_type(:string) }
-    it { is_expected.to have_db_column(:last_sign_in_ip).of_type(:string) }
-    it { is_expected.to have_db_column(:first_name).of_type(:string).with_options(null: false) }
-    it { is_expected.to have_db_column(:last_name).of_type(:string).with_options(null: false) }
-    it { is_expected.to have_db_column(:phone_number).of_type(:string) }
-    it { is_expected.to have_db_column(:active).of_type(:boolean).with_options(null: false, default: true) }
-    it { is_expected.to have_db_column(:role_id).of_type(:integer).with_options(null: false) }
-    it { is_expected.to have_db_column(:gender).of_type(:string).with_options(null: false) }
-    it { is_expected.to have_db_column(:deleted_at).of_type(:datetime) }
-    it { is_expected.to have_db_column(:stripe_customer_id).of_type(:string) }
+    it { should have_db_column(:email).of_type(:string).with_options(null: false, default: "") }
+    it { should have_db_column(:encrypted_password).of_type(:string).with_options(null: false, default: "") }
+    it { should have_db_column(:reset_password_token).of_type(:string) }
+    it { should have_db_column(:reset_password_sent_at).of_type(:datetime) }
+    it { should have_db_column(:remember_created_at).of_type(:datetime) }
+    it { should have_db_column(:sign_in_count).of_type(:integer).with_options(null: false, default: 0) }
+    it { should have_db_column(:current_sign_in_at).of_type(:datetime) }
+    it { should have_db_column(:last_sign_in_at).of_type(:datetime) }
+    it { should have_db_column(:current_sign_in_ip).of_type(:string) }
+    it { should have_db_column(:last_sign_in_ip).of_type(:string) }
+    it { should have_db_column(:first_name).of_type(:string).with_options(null: false) }
+    it { should have_db_column(:last_name).of_type(:string).with_options(null: false) }
+    it { should have_db_column(:phone_number).of_type(:string) }
+    it { should have_db_column(:active).of_type(:boolean).with_options(null: false, default: true) }
+    it { should have_db_column(:role_id).of_type(:integer).with_options(null: false) }
+    it { should have_db_column(:gender).of_type(:string).with_options(null: false) }
+    it { should have_db_column(:deleted_at).of_type(:datetime) }
+    it { should have_db_column(:stripe_customer_id).of_type(:string) }
   end
 
   describe "db_indexes" do
-    it { is_expected.to have_db_index(:active) }
-    it { is_expected.to have_db_index(:email).unique }
-    it { is_expected.to have_db_index(:first_name) }
-    it { is_expected.to have_db_index(:last_name) }
-    it { is_expected.to have_db_index(:gender) }
-    it { is_expected.to have_db_index(:role_id) }
-    it { is_expected.to have_db_index(:reset_password_token).unique }
-    it { is_expected.to have_db_index(:stripe_customer_id).unique }
+    it { should have_db_index(:active) }
+    it { should have_db_index(:email).unique }
+    it { should have_db_index(:first_name) }
+    it { should have_db_index(:last_name) }
+    it { should have_db_index(:gender) }
+    it { should have_db_index(:role_id) }
+    it { should have_db_index(:reset_password_token).unique }
+    it { should have_db_index(:stripe_customer_id).unique }
   end
 
   describe "associations" do
     describe "belongs_to" do
       # Use `without_validating_presence` with `belong_to` to prevent the
       # matcher from checking whether the association disallows nil (Rails 5+ only).
-      # This can be helpful if you have a custom hook that always sets
-      # the association to a meaningful value:
-      it { is_expected.to belong_to(:role).inverse_of(:users).without_validating_presence }
+      # This can be helpful if you have a custom hook that always sets the association to a meaningful value:
+      it { should belong_to(:role).inverse_of(:users).without_validating_presence }
     end
 
     describe "has_many" do
-      it { is_expected.to have_many(:orders).inverse_of(:user).dependent(:restrict_with_exception) }
+      it { should have_many(:orders).inverse_of(:user).dependent(:restrict_with_exception) }
     end
   end
 
@@ -72,7 +69,7 @@
     end
 
     describe "inclusion" do
-      it { is_expected.to validate_inclusion_of(:gender).in_array(User.genders.keys) }
+      it { should validate_inclusion_of(:gender).in_array(User.genders.keys) }
     end
 
     describe "length" do
@@ -98,24 +95,28 @@
     describe "format" do
       subject { build :user }
 
-      it "first_name accepts a valid value" do
-        subject.first_name = "John"
-        expect(subject).to be_valid
-      end
+      describe "first_name" do
+        it "accepts a valid value" do
+          subject.first_name = "John"
+          expect(subject).to be_valid
+        end
 
-      it "first_name does not accept an invalid format" do
-        subject.first_name = "John-1"
-        expect(subject).to be_invalid
+        it "does not accept an invalid format" do
+          subject.first_name = "John-1"
+          expect(subject).to be_invalid
+        end
       end
 
-      it "last_name accepts a valid value" do
-        subject.last_name = "Doe"
-        expect(subject).to be_valid
-      end
+      describe "last_name" do
+        it "accepts a valid value" do
+          subject.last_name = "Doe"
+          expect(subject).to be_valid
+        end
 
-      it "last_name does not accept an invalid format" do
-        subject.last_name = "Doe-1"
-        expect(subject).to be_invalid
+        it "does not accept an invalid format" do
+          subject.last_name = "Doe-1"
+          expect(subject).to be_invalid
+        end
       end
     end