The Liberal Democrats offered a "copper-bottomed guarantee" that mail will continue to be delivered anywhere across the United Kingdom at the same price.
There had been fears that the universal service obligation could be under threat after Business Secretary Vince Cable threw his weight behind a report on the future of the Royal Mail, which said significant amounts of private capital were needed to keep the business afloat.
Last week the TUC backed a campaign by the Communication Workers Union to oppose plans to privatise the business, which claims hundreds of post offices will close and the universal postal service will end if it is sold off.
But at the Liberal Democrat's annual gathering in Liverpool, Tavish Scott, leader of the Scottish Liberal Democrats said: "I want to be very clear what a future postal service for Scotland must be. Yes explore how best to keep Royal Mail solvent, to stabilise the business, to modernise the network.
"But three conditions, three Scottish conditions apply to reform.
"So conference, I am delighted to tell you that I have secured an agreement from our Business Secretary, Vince Cable.
"He has confirmed today, firstly the universal service obligation is a copper-bottomed guarantee. Posties delivering the same letter at the same price, anywhere in the UK, whether it is Kensington or Kyelachlan.
"Second, Labour's Post Office closure programme, which saw 1,000 close in Scotland will be halted. That is because we know that in many areas Post Offices are part of the community.
"Thirdly, posties will be allowed shares in their company."
