Sub-postmasters in Suffolk claim they were "bullied and blackmailed" as part of a second post office scandal which they said was "as big as Horizon".
Jerry Brown, 68, and Adrian Straatman, 63, have hit out at the extent to which the Post Office’s Network Transformation (NT) project resulted in "dire consequences".
The scheme, which started in 2012, resulted in contract changes which saw sub-postmasters paid per transaction - rather than receiving a fixed salary.
The National Federation of Sub Postmasters, external has since launched a campaign calling for a full review into the project. The Department for Business and Trade has promised a response.
At the time of the NT being implemented, sub-postmasters say they were told that the Post Office would become the "front office" for new services.
These included selling train and cinema tickets, giving out fishing licences, accepting speeding fine payments, and giving out Department for Work and Pension forms.
It was suggested that, because the postmasters would complete more transactions, they would in-turn generate more income for themselves under the new plan.
But, according to Mr Brown, of Hadleigh Post Office, these new services didn’t quite materialise and, along with many of the more traditional offerings, moved online.
'Devastating effect'
"We were promised lots of jam for losing the allocated monthly payment but they were telling their departments not to give it to us because it had to go online," he said.
"That has had an absolutely devastating effect on us and our income and caused horrendous problems for postmasters.
"My generated income has dropped by 30% compared to what it was in 2012 - in real terms that means it has halved with inflation, and that’s unsustainable.
"I’m a pensioner and it’s my pension that is helping to keep the post office open - without that I don’t know where we’d be.
"We have been stripped of an income we should have always had – you are talking in the hundreds of thousands of pounds lost compared to what it would have been.
"This is different, but in many ways it is as big a scandal as the Horizon scandal."
Mr Straatman, who was the sub-postmaster at the now-closed Yoxford Post Office, refused the NT contract before leaving to work in hospitality.
He said: "I could barely make a living as it was, and with it being commission-only it was going to be impossible for me to carry on with it, so we left the premises.
"My average income would have been an average of £160 a week [under the new contract] - it was a non-starter really.
"It just didn’t make sense why postmasters were earning so little money in the first place – we were doing a service for the community as opposed to making a living.
"Postmasters need to be paid at least minimum wage.
"If I was still doing it now I would have lost my house because it was a complete disaster for us financially."
Bosses at the Department for Business and Trade said they were aware of a campaign against the Network Transformation scheme.
A spokesperson said: "We will respond to the National Federation of Sub Postmasters' letter soon."