"I am having real trouble getting charities to accept that this is a problem," said Laura Thomas, policy officer at the institute. "It could be the death knell for postal direct mail."
Although she emailed and spoke at length to fundraising charities to explain the ramifications of the new pricing and impress upon them the need to add their voice to the institute's response to the consultation, Thomas said not one charity had responded.
And just seven people turned up to the workshop called 'Great things come in small packages', hosted by David Dale, head of size-based pricing at Royal Mail, at the institute's National Convention on Wednesday.
"I don't think the sector understands that when it comes to legislation such as this, which affects charities, we are listened to - and the more voices we represent, the more powerful we can be," said Thomas.
"It may be that charities have looked at it and think it will be all right. But if that's so, I wish they would tell us."
The regulator, Postcomm, is due to announce the new policy on 1 August and is widely expected to implement it next April.
Robert Mayes, director of public affairs at direct marketing agency WWAV Rapp Collins, who has been lobbying hard against the changes, said charities should be ready to test new creative. "If your best-performing pack is of non-standard format, then you are going to have to pay significantly more in postage," he said. "So it's imperative that charities are ready to start testing as soon as the regulator announces its decision next month. If you send pens in your packs, this will cost more too - an extra 14p per item in the case of one of our charities."
The consultation has now closed, but Thomas is still urging charities to write to the Royal Mail, which is currently considering proposals to mitigate the effects, such as delaying the start of the new regime.
Royal Mail has suggested giving a rebate to organisations whose postal costs rise by more than 50 per cent and which spend more than £100,000 a year. But Thomas said this would only help the biggest charities and would be only an interim buffer.