A new contract offered by An Post to postmasters includes a gagging order banning post office staff from criticising the semi-State company.
Lawyers acting for postmasters described the clause as "draconian".
Postmasters fear the seven year contracts offered to 690 postmasters threatens long-term viability because they have not been offered a right to automatic renewal.
Another 390 postmasters have been offered an exit package or a chance to stay on their current contract.
Members of the Irish Postmasters Union are balloting until Wednesday on whether to accept the new deal.
The new 40-page contract, seen by the Sunday Independent, is accompanied by a letter from An Post retail managing director Debbie Byrne promising 95pc of the population will be within 15km of a post office.
The contract warns that "a postmaster or postmaster's employee must make no criticism of the company or company customers, whether corporate and retail".
Chairman of the Independent Postmasters Group Tom O'Callaghan said he is concerned about the sustainability of rural post offices.
He said it is vital new services are introduced to boost business and make running a post office more secure.
"We need to see an extension of the Social Welfare contract and the roll out of new banking services and public services, such as car tax and other payments to really secure our futures."
A spokeswoman for An Post said no post offices will be forced to close as part of the company's attempts to modernise. An Post will roll out foreign exchange cash and card services in 650 outlets.