Can Allow charges only if one card is marked as default

app/assets/javascripts/darkswarm/controllers/credit_cards_controller.js.coffee

@@ -20,3 +20,7 @@ angular.module('Darkswarm').controller "CreditCardsCtrl", ($scope, $http, Credit
     ).finally ->
       window.location.reload()
 
+
+  $scope.hasOneDefaultSavedCards = () ->
+    $scope.savedCreditCards.length > 0 && $scope.savedCreditCards.some((card) -> card.is_default)
+

app/views/spree/users/_authorised_shops.html.haml

@@ -12,5 +12,6 @@
         name: 'allow_charges',
         ng: { model: 'customer.allow_charges',
           change: 'customer.update()',
+          disabled: "!hasOneDefaultSavedCards()",
           "true-value" => "true",
           "false-value" => "false" } }