TNT Post will now collect and sort mail from Brightsource, which was formerly Target Direct Print, before passing it on to Royal Mail for final mile delivery.
The deal applies to Brightsource's Fasttrack service, which was launched in 2006 with DHL as sole supplier of upstream handling and is of particular appeal to charity clients.
The service minimises charity clients' exposure to VAT because it integrates print and postal management.
Under single supply VAT regulations, if a direct mail format is zero-rated, then other ancillary services (such as postage) can also be zero-rated if they are supplied by the same supplier.
Fasttrack is regularly used by more than 90 clients, including Unicef, British Heart Foundation and the RSPCA, and is responsible for mailing 10m items per month.
Brightsource picked TNT not only because its service was cost effective but also because it approves of its environmental credentials, according to Brightsource managing director Peter Frings.
Frings said: "Like TNT Post, Brightsource is looking to build on the success of its own carbon footprinting service by enabling some of the UK's largest fundraising marketers to accurately preview and control carbon emissions on a campaign-by-campaign basis.
"We look forward to working together with TNT Post to offer our customers a truly environmentally responsible service, whilst providing fantastic value for money."