Britons have just twelve days to apply for shares in Royal Mail, starting from today.
The share sale, at a price between 260p and 330p, comes as ministers revealed the business would float on the stock market in October.
By comparison, a first class stamp costs 60p and a second-class stamp costs 50p. Royal Mail confirmed that the price of first and second class stamps is set to rise in line with RPI inflation over each of the next three years.
The sale would come as the Communication Workers Union is set to decide whether to launch days after the stock market flotation.
The sell-off would see Royal Mail given a market valuation of up to £3.3 billion. Individuals have a deadline of October 8 to apply for shares, with a market debut set for October 11 and the business to trade unconditionally on October 15.
CWU members are beginning today to vote on whether to take industrial action in opposition to the plan, with the ballot set to conclude mid-October.
CWU general secretary Billy Hayes said: "It seems remarkable that the prospectus is being issued on the same day that postal workers are being sent ballot papers for strike action."
"Royal Mail is profitable and can continue to be successful in the public sector. The sale is driven by political dogma, not economic necessity, and postal workers and the CWU will continue to fight to save services as well as defend their terms and conditions."
Ministers said in a statement that 10% of the shares would be given to around 150,000 "eligible UK-based Royal Mail employees". The coalition plans to sell between 40.1% and 52.2% of the Royal Mail.
Business Secretary Vince Cable said: "This will give Royal Mail access to the private capital it needs to modernise, as envisaged under successive governments and enshrined in law by Parliament two years ago."