Post Office Ltd is to close two main postal offices in north and south Wales in new franchise deals with WH Smith.
The move will affect jobs, says the Communication Workers Union
The post offices at Bangor, Gwynedd and Cwmbran, Torfaen, will close at the beginning of April.
Post Office Ltd said the relocation of services to the WH Smith shops would "safeguard and enhance" services.
But the Communication Workers Union (CWU) said the franchising system had cost "thousands" of post office jobs since it began five years ago.
The decision to close the current premises and move the postal service to WH Smith in Bangor and Cwmbran was taken after a six-week consultation.
The new Bangor post office will be at 264 High Street, some 260 yards (237m) from the current site in Deiniol Road.
Post Office Ltd said the relocation followed successful trials of six post offices in WH Smith stores, which will be extended to a further 32 locations this year.
WH Smith area manager Ian Rowley said it was good news for customers in both Bangor and Cwmbran.
"Our highly-trained staff are committed to delivering an enhances service with longer opening hours from a convenient location that is fully accessible to all customers," he added.
A CWU spokeswoman said as well as costing jobs, there was also an issue with the franchising system of loss of pay, holiday and long-service entitlement to postal workers.