There has been a significant impact on thousands of businesses from the current dispute between An Post management and staff.
The small business organisation, ISME, estimates that 20,000 small and medium-sized enterprises depend on An Post each day.
The disruption to services will impact most severely on business in Dublin city and county.
Mr Mark Fielding, chief executive of ISME, said some firms will have cash-flow problems.
"Cash flow becomes a problem when businesses cannot perform their usual banking dealings and payments are late.
"Firms in the competitive sector do not have much leeway for cash being late," he said.
He said if even 1 per cent of the 20,000 firms using An Post experienced cash-flow problems there could be job losses.
Firms have little option but to wait until the service returns. Legislation prevents anyone but An Post delivering standard-size mail. This part of the market is known as the reserved area.
Private firms are allowed to post larger items and firms such as DL and Spring Global Mail (which is linked to TNT) compete with An Post in this segment of the market.
Regarding standard mail, companies can use a courier or what is known as "document exchange" service.
This service allows companies to post letters to other firms. But it is strictly a business-to-business service and posting to residences is not allowed.
Mr Kevin Galligan, director of DX Ireland, which provides the document exchange service for an annual fee, said 3,500 firms were members and DX was now shifting up to 30,000 pieces of mail a night.
The service guarantees next day delivery (by 9 a.m.) once letters are left in the 240 document exchange boxes in the Republic by 5 p.m. the previous evening.
He said he expected volume to increase because of the dispute, although the company's main commitment was to its existing customers.
- Emmet Oliver