WWAV Rapp Collins, the UK's largest direct agency, has written to Gillian Wilmot, Royal Mail's managing director of business and consumer marketing, accusing it of proposing structures that would "stifle creativity and seriously damage the direct mail medium".
Royal Mail is currently in a consultation period over the proposed changes that would see new charges being based on the size of mailings rather than weight. It is considering basing price on three different size categories.
WWAV and other agencies are worried this will stifle creativity.
Chris Gordon, WWAV's chairman and group chief executive, said: "Royal Mail has sought to position itself as a responsible media owner, making significant efforts to promote the direct mail medium and creativity within the medium. Clients and agencies are striving to improve the quality of direct mail and to achieve standout by using innovative formats. Yet these pricing proposals, if adopted, would simply incentivise standardisation, penalise innovation and, as such, increase consumer apathy and produce a less effective medium."
The consultation process on the proposals, which WWAV said could more than double the cost of some mailings, ends on 28 June. Postcomm, the mail regulator, will then review the situation.
While the changes will bring advantages to Royal Mail - building its revenue stream while bringing its pricing structures into line with other European countries - some argue the long-term effect on the medium will be negative.
David Robottom, the director of development and postal affairs at the DMA, said: "A key determinant of cost under the proposals will relate to size. This will put the industry into a straightjacket because it will be tied to standard sizes ... it's standardisation by stealth."
Steve Aldridge, the creative partner at Partners Andrews Aldridge, said: "The very people who should be championing creativity are the ones who are cutting its legs off. Bigger format work will become more prescriptive and will require larger budgets. This is a tax on creativity."
However, Simon Marshall, the managing director of Publicis Dialog, said: "I don't think this is too big an issue. Unusually shaped or larger mailings tend to be of higher unit cost."
A Royal Mail spokeswoman said: "We know there are concerns about creativity but there would be opportunities under our proposals for some of our customers to save substantial sums of money. We feel there is still room for creativity in a system which takes size into account for pricing. But one of the reasons for the consultation is for us to listen and understand the different views on this."