An animal welfare group whose freepost was targeted with bulky packages and empty envelopes will pay just £2,000 of the £500,000 postal cost, it has said.
The League Against Cruel Sports had its freepost address targeted
The League Against Cruel Sports said Royal Mail had agreed it would be liable only for the cost of mail sent before it cancelled the address.
It received mail including excrement and bricks, after an e-mail urged hunt supporters to abuse the address.
The league said it was delighted the "spiteful" campaign had failed.
It said the campaign followed an e-mail to pro-hunters, citing the support of the Countryside Alliance, which urged them to target the freepost address.
The Royal Mail depot in Poole, Dorset, called in the bomb squad after bulky packages arrived addressed to the league.
The parcels were later found to contain bricks.
The police, CID and Royal Mail fraud investigators launched an inquiry as the league was swamped by vanloads of mail.
A Royal Mail spokesman said: "The freepost that was operated by the league was cancelled in December and since then any items that have been sent to the freepost have been returned to sender."
A spokesperson for the league said: "We will carry on exposing the hunters' illegal activities, their cruelty to animals, and the new depths to which they sink in their desperation to carry on tormenting wildlife.
"They have wasted the time and resources of the bomb squad, the CID, and the Royal Mail, through their petty and spiteful campaign.
"Next time the price of posting a letter goes up, you have the hunters partly to thank for that."
The spokesperson said donations had increased due to "disgust" at the tactics used.
"This was a nasty and low-down campaign waged by the hunters, which has spectacularly backfired."
The league alleged senior figures in the Countryside Alliance and the Masters of Foxhounds Association (MFHA), were involved in circulating the e-mail.
However, both bodies denied organising the campaign.
The Countryside Alliance said: "We know that supporters of hunting have got in to this but it's nothing to do with the Countryside Alliance.
"We were alerted to it but we certainly haven't advertised it because in the past it has happened to us and it's not something that we advocate."
Meanwhile, MFHA director Alastair Jackson said he had "no idea" where it started.
"All sorts of different people have heard from different sources but once something like that starts it gets a life of its own.
"My wife was e-mailed about it, everyone in the countryside was e-mailed by someone or other. So many people heard and passed it on to their friends, I have simply no idea."
The league also alleged that City law firms, estate agencies and chartered surveyors were involved in the mail campaign - as well as the wife of a Buckingham Palace employee.
Meanwhile, TV presenter Jeremy Clarkson used his national newspaper column to print the Freepost address and urge people to send empty envelopes.