In the pilot 1,000 ABC1 men aged 24-35 were sent a light blue cardboard box, branded "Matter", containing objects including soap crayons from Nissan, tubs of Play-Doh from Sony Bravia and a sweatband from Nintendo Wii as well as a Clubhouse Lounge offer from Virgin Atlantic.
Royal Mail claims that pure word of mouth from recipients generated a flood of people signing up to receive future "Matter boxes" at , meaning it now has 30,000 subscribers in the UK and 10,000 abroad.
It is offering brands a place on a category exclusive basis in the next mailing, which will go out to the 30,000 UK subscribers on the last Saturday in October, for £14,000.
Brands already booked in include a large drinks brand and a high-profile DVD release, according to Fraser Chisholm, head of media productions at Royal Mail.
Royal Mail has created the offering by teaming up with Matter Media, which created the idea and advises marketers about producing an object likely to appeal to consumers. The box bears some unobtrusive Royal Mail branding.
The mail operator has carried research with recipients of the first box and secured endorsements from the marketing community.
Rory Sutherland, vice-chairman of Ogilvy Group UK, said: "In my opinion this is one of the biggest ideas in DM for years."
Bill Gosbee, a marketer at Virgin Atlantic, said it would "definitely take part in future boxes...The halo of other exciting brands ensures that everyone engages with your stuff, even if they subsequently pass it on to someone else."