The majority of our domestic letter and large letter postage is sent by Mailmark® , and where this is not possible, we will always look for the next most cost-effective service. Mailmark® is the most cost-effective way to send letters and large letters, whether it’s Business Mail or Advertising Mail. Mailmark® barcodes are added to mail pieces which enable the items to be scanned and sorted faster by Royal Mail, which in turn reduces the postage cost.