design

main.rb

module PaymentMethods
  class Base
    def request_url(params)
      raise 'Not implemented'
    end

    def response_notification
      raise 'Not implemented'
    end
  end

  class Skrill < Base
    def request_url(params)
      #some implementation
    end

    def response_notification
      #some implementation
    end
  end

  class Obt < Base
    def request_url(params)
      #some implementation
    end

    def response_notification
      #some implementation
    end
  end
end

module PaymentProvider
  def self.for(type)
    if known_payment_method?(type)
      klass = type.classify.constantize
      "PaymentMethods::#{klass}".constantize.new
    else
      raise "Unknown payment method '#{type}'"
    end
  end
end

PaymentProvider.for(:skrill).request_url(params)