The Royal Mail has warned charities that they should not assume they are entitled to refunds if donors put stamps on Freepost envelopes supplied to send in donations.
Postage is often paid twice on mailed donations because Royal Mail machinery recognises and charges the Freepost number irrespective of whether there is a stamp on the envelope.
In a statement, the Royal Mail said that if charities proved they had been charged for stamped Freepost envelopes, it gave refunds "as a goodwill gesture". But a spokesman said the Royal Mail had never authorised boxes printed on envelopes that request stamps - these had been invented by fundraisers.
Donors used stamps because they thought charities would not be charged if they did, but the Freepost service did not have this optional facility, he said. The Royal Mail has told charities they should recognise they are not automatically entitled to a refund if donors use stamps, but it is not planning to stop giving refunds.
Many charities, including Oxfam, Cancer Research UK, the Salvation Army and the RSPB, still print a box on Freepost envelopes asking for a stamp. A sample of 20 charities' recent direct mail packs showed that 10 claimed adding a stamp "would save the charity money", and a further five said it "could" do so.
A Salvation Army spokeswoman said around half of the 200,000 respondents to a recent direct mail appeal had used a stamp. This would let the charity reclaim between £30,000 and £39,000, depending on whether the stamps were first or second class.
Nick Georgiadis, head of marketing at Cancer Research UK, said the charity did claim refunds, but he could not tell how much money it saved.
Pam Knight, the director of fundraising and communications at St Margaret's Somerset Hospice, said 2,124 respondents to a recent appeal that asked donors to use stamps on Freepost envelopes did so, and 627 did not. She said Royal Mail had never made it clear that charities would still be charged if donors used stamps.
David Burrows, head of fundraising at direct marketing agency TDA, said: "Even if it doesn't raise much money, it makes charities look like they care about every penny."