responding to the first commenter's idea from http://codeulate.com/2012/07/depend-upon-abstractions/

gistfile1.txt

# app/models/user.rb
class User
  
  attr_reader :payment_gateway
  
  def initialize
    @payment_gateway = PaymentGateway::Braintree.new
  end
  
  def charge_for_subscription
    payment_gateway.charge_for_subscription
  end

  def create_as_customer
    payment_gateway.create_customer(user.name)
  end
end

# app/models/refund.rb
class Refund
  def process!
    PaymentGateway::Braintree.new.refund(self)
  end
end

# New file: lib/payment_gateway.rb
class PaymentGateway

  SUBSCRIPTION_AMOUNT = 10.to_money
  
  class Braintree

    def charge_for_subscription
      Braintree.charge(SUBSCRIPTION_AMOUNT)
    end
  
    def create_customer(customer_name)
      Braintree.create_customer(customer_name)
    end
  
    def refund(refund)
      Braintree.refund(refund.order_amount, refund.user_braintree_id)
    end
    
  end
  
end

Ok, nice. I could see decoupling the payment vendors (Braintree and others) from the PaymentGateway if we did go with injecting Braintree into the PaymentGateway as you suggested in your comment. Would you take it that far in practice? Thanks for explaining!