A number of Post Office branches across Northern Ireland have been unable to serve customers due to computer problems.
Staff were unable to connect to the organisation's Horizon computer system for several hours on Wednesday afternoon.
The connectivity problem meant they were forced to turn customers away from some branches.
In total, 2,000 branches were affected across the UK.
In Hillsborough, County Down, the problem lasted three days.
May Moorehead runs a watch and clock supply business in the town. Speaking to BBC News NI, she said: "I went to the post office in Hillsborough on Monday and was told their system was down and they couldn't send my packages.
"Eventually I got it sent through the post office in Lisburn, although I had to explain to my customers that the stock would be arriving a day late.
"Then on Wednesday, I went to Lisburn to post as I knew Hillsborough was still having problems, but then I was told Lisburn was down too."
Post Office branches have a computer system called Horizon, which which thousands of sub-postmasters use.
The Post Office said in a statement: "We're really sorry for any inconvenience that the connectivity issues at some of our branches caused yesterday. The issue was resolved within a few hours, and our branches are now back to business as usual.
"Hillsborough Post Office experienced a separate and isolated issue affecting service since Monday, for which we apologise, although we are pleased to say this is now sorted and the branch is open as normal."
A spokesperson at the National Federation of SubPostmasters said its members have suffered financially because of the problems.
"Sub-postmasters only get paid if they are serving customers so any downtime means they are out of pocket, and people are unable to send their mail.
"In the past two weeks we've had two instances, just under a quarter of the network was affected earlier in the month, and yesterday the whole network was down for a couple of hours.
"The Post Office uses a nationwide computer system to make sure all items are tracked correctly before being sent. If this suddenly stops working then it means potential delays to your parcel across all depots in the UK."