Royal Mail has conceded that charities could be charged multiple times for using Freepost envelopes after a small charity was charged six times to receive one Freepost letter on two separate occasions.
Charities use Freepost envelopes so their supporters do not have to pay to send letters or donations. They are required by Royal Mail to reimburse the cost of sending the letters.
Michael Jamison, operations manager at disability charity Through the Roof, said he regularly applied for refunds from Royal Mail when the charity had been overcharged. He said his invoices showed the charity was charged once every time an envelope was franked by Royal Mail, and many were stamped in multiple places.
Jamison said the charity had consistently been reimbursed for about two-thirds of what it was charged by Royal Mail. Sometimes this was because donors had put stamps on Freepost envelopes, for which the Royal Mail reimburses charities, and some was due to repeat charging, he said.
A Royal Mail spokesman said it was possible for envelopes to be charged repeatedly if they were put through machines more than once.
"If we are made aware that there has been a duplication in our charging, we will, of course, refund any overpayment," he said.
Jamison said his charity did not use many Freepost envelopes, so it could keep a close check on them. "If bigger charities aren't watching out for this, it could be costing them a lot of money," he said. "The sector as a whole could be losing millions."
In the past month, the Epsom-based charity had received 17 Freepost envelopes, he said, of which nine had gone through the system once and eight had gone through multiple times.
"It's costing us in staff time to keep asking Royal Mail for refunds, but it's a point of principle," he said. "When I'm being charged six times to receive one donation, it's excessive."
Pam Knight, director of fundraising and communications at St Margaret's Somerset Hospice, said that about half of the Freepost envelopes the charity had received in response to its latest fundraising appeal had more than one mark on them from Royal Mail. She said she was not sure whether the charity had been repeatedly charged for these.