def credit(amount)
      credit = Credit.new(:amount => amount)
      result = api_request(:post, "/subscribers/#{self.customer_id}/credits.xml", :body => credit.to_xml)
      self.store_credit = (self.store_credit || 0) + amount
      true
    end
def credit(amount)
      credit = Credit.new(:amount => amount)
      result = api_request(:post, "/subscribers/#{self.customer_id}/credits.xml", :body => credit.to_xml)
      self.store_credit = (self.store_credit || 0) + amount
      true
    end
Give a subscriber a credit (or reduce credit by supplying a negative value (more)
 POST /api/v4[short site name]/subscribers/[subscriber id]/credits.xml