spree_paypal_express/app/models/spree/billing_integration/paypal_express.rb

class Spree::BillingIntegration::PaypalExpress < Spree::BillingIntegration
  preference :login, :string
  preference :password, :password
  preference :signature, :string
  preference :review, :boolean, :default => false
  preference :no_shipping, :boolean, :default => false
  preference :currency, :string, :default => 'USD'
  preference :allow_guest_checkout, :boolean, :default => false

  def provider_class
    ActiveMerchant::Billing::PaypalExpressGateway
  end
end