foo.rb

class User
  def charge_for_subscription
    PaymentActions.new(user, BraintreeGemSomethingSomething.new).charge_for_subscription
  end
end

class PaymentActions
  def initialize(user, braintree)
    @user, @braintree = user, braintree
  end

  def charge_for_subscription
    @braintree.create_subscription(@user)
  end
end