code: function(couponIds) {
                  console.log('ecInterface.order.prepare');
  
                  corbel.validate.value('id', this.id);
                  return this.request({
                      url: this._buildUri(this.uri, this.id, '/prepare'),
                      method: corbel.request.method.POST,
                      data: couponIds
                  });
              }
docs: Prepares the order, required to checkout
  @method
  @memberOf corbel.Ec.OrderBuilder
  @param {string[]} couponIds                Array of String with the coupons ids to prepare the order
  @return {Promise}                          Q promise that resolves to undefined (void) or rejects with a
  {@link SilkRoadError}