The milestone has been reached nine months after the initial launch of the initiative, which offers tariff discounts on bulk direct mail that adheres to a range of green criteria.
Matthew Neilson, head of environmental solutions at Royal Mail, said: "There has been tremendous interest in Sustainable Mail and we are delighted that so many brands have signed up to the service.
"Reaching 100 million items in the first nine months is an impressive and important milestone for us."
The scheme has around 75 customers, including BHS, EDF Energy, National Grid and NS&I.
In order to qualify for the discount, companies need to meet three criteria: improved sustainability, minimised waste and increased recyclability.
The cost reduction available for Sustainable Mail with Mailsort 120 or 700 is 2% for entry level or 4.7% for intermediate level.
At entry level for Mailsort 1400, the reductions range from 2% to 1.05%, depending on item format and whether there is direct or residue selection. For intermediate level, they range from 4.7% to 2.55%.
