Postcomm, the postal industry regulator, is launching a consultation process this month on Royal Mail's proposals to replace weight-based pricing with a new model that will charge a premium on any post that falls outside its standard envelope size.
Charities will have three months to respond, and the Institute of Fundraising is urging the voluntary sector to take advantage of this "last chance" to fight against the proposals.
Subject to the consultation, size-based pricing could be introduced in September 2005. Although the new costs have yet to be confirmed, the National Trust estimates that size-based pricing could cost it up to £40,000 a year.
The charity recently conducted a direct-mail recruitment campaign, which tested leaflets altered to fit the Royal Mail standard size, and found that response rates dropped by 10 per cent.
"Overall this proposal is bad news for us," said Berry Darcy, direct marketing manager at The National Trust. "Although the new prices as they stand will mean gains and losses in different areas, a lot of our direct mail, such as our supporter magazines and property guide, are outside the standard sizing, and this will push costs through the roof."
Joel Voysey, supporter recruitment manager at Amnesty UK, believes that the introduction of sized-based pricing could hamper charities' ability to use creative and engaging direct mail.
"These new pricings could cost charities a fortune and stop us producing the quality of material that currently induces people to start supporting good causes," he said.
"Our current packs would be too thick and push us up into a higher price band, so we'll have to think very carefully about what we can do to get around this."
Royal Mail says the new pricing structure will increase efficiency and lower the costs of sorting and distributing post. It insists it will result in an overall decrease in the cost of sending mail, and will also be more environmentally friendly.