Precisely how many independent retailers will benefit and how contracts with other machine providers will work remains unclear.
But the move is being heralded as a victory for the sector, with the Post Office installing 300 free ATMs in branches that are currently without one from the autumn.
Another 700 will replace fee-charging machines in other branches in the next 18 months in partnership with the Bank of Ireland.
The Post Office said it hoped to replace more machines levying surcharges of up to £1.50 on every transaction as existing contracts finish.
In addition, sub-postmasters, many of whom are independent retailers, will be free to sign deals with other providers. An ACS spokesman said: “This appears to be a step towards greater retailer choice. We will wait to see how this develops.”
The ACS presented an official complaint to the Office of Fair Trading in November about the restrictive contracts that the Post Office operates with independent stores.
The organisation opposed contracts tying retailers in to certain cash machine providers and requiring them to pass on a portion of margin on services such as the National Lottery.
The ACS argued such business terms were anti-competitive.
The OFT’s investigation into the issue is ongoing, but independents told The Grocer that their victory seemed increasingly likely.
“We have met the Post Office recently and it is talking in terms of when it will lose the case, not if,” said one insider.
The Post Office refused to comment on what it called “speculation”.