It is a requirement of the law that Royal Mail treats articles for the blind as first class post delivered free of charge. There are, of course, rules to ensure that this is not abused. Basic information only can be found at www.RoyalMail.com and searching for Articles for the Blind.
The full rules can be found at http://www.royalmail.com/you-home/your-community/articles-blind/faqs
TNF recommends that care be taken to ensure that address labels used in delivery pouches are clear and that post-codes are properly displayed. By doing this the likelihood of next day delivery will be greatly improved.