Add Venmo as payment option on checkout

Resolves issue/ticket:
- solidusio#136

Setting the payment method preference `enable_venmo` to true will now
show a Venmo payment option on checkout for customers whom it is
available to (currently only US).

README.md

@@ -120,6 +120,16 @@ With product and cart page checkout, the user is directed to the checkout confir
 
 PayPals API does not allow for admin-side payments. Instead, backend users taking payments for customers will need to use the PayPal Virtual Terminal to take payments. [More info is available on the PayPal website.](https://www.paypal.com/merchantapps/appcenter/acceptpayments/virtualterminal?locale.x=en_US)
 
+## Venmo
+You can enable Venmo to appear as a funding source/a button below PayPal's button, by updating the Solidus PayPal Commerce `Payment Method`'s preference `enable_venmo` to `true`. See more about preferences below.
+
+Please note, there are prerequisites you must meet for Venmo to be available to your store and customers. [Read more here](https://developer.paypal.com/docs/business/checkout/pay-with-venmo/#eligibility).
+
+## Configuration
+The easiest way to change the `Payment Method`'s preferences is through admin: `Settings > Payments > "PayPal Commerce Platform" > Edit`.
+
+See more about preferences [here](https://guides.solidus.io/developers/preferences/add-model-preferences.html#access-your-preferences)/
+
 ## Development
 
 ### Testing the extension

app/models/solidus_paypal_commerce_platform/payment_method.rb

@@ -12,6 +12,7 @@ class PaymentMethod < SolidusSupport.payment_method_parent_class
     preference :display_on_cart, :boolean, default: true
     preference :display_on_product_page, :boolean, default: true
     preference :display_credit_messaging, :boolean, default: true
+    preference :enable_venmo, :boolean, default: false
 
     def partial_name
       "paypal_commerce_platform"
@@ -73,6 +74,7 @@ def javascript_sdk_url(order: nil, currency: nil)
       }
 
       parameters[:shipping_preference] = 'NO_SHIPPING' if step_names.exclude? 'delivery'
+      parameters['enable-funding'] = 'venmo' if options[:enable_venmo]
 
       "https://www.paypal.com/sdk/js?#{parameters.to_query}"
     end

spec/models/solidus_paypal_commerce_platform/payment_method_spec.rb

@@ -113,6 +113,22 @@ def Struct(data) # rubocop:disable Naming/MethodName
         expect(url.query.split("&")).to include("components=buttons")
       end
     end
+
+    context 'when enable_venmo is false' do
+      before { paypal_payment_method.preferences.update(enable_venmo: false) }
+
+      it 'does not include the "enable-funding=venmo" parameter' do
+        expect(url.query.split('&')).not_to include('enable-funding=venmo')
+      end
+    end
+
+    context 'when enable_venmo is true' do
+      before { paypal_payment_method.preferences.update(enable_venmo: true) }
+
+      it 'includes "enable-funding=venmo" as a parameter' do
+        expect(url.query.split('&')).to include('enable-funding=venmo')
+      end
+    end
   end
 
   private