The union believes job losses of crown office workers who are specifically trained to explain such changes to customers will put added pressure on less experienced retailers.
CWU national official Andy Furey said it was "appalling" that the Post Office was pressing ahead with closures and job cuts at the same time as it was implementing changes in the business.
He said: "By systematically cutting jobs in the Post Office, franchising out the service to inexperienced retailers and downsizing the network, Royal Mail has bitten off the helping hand it needs to implement this business change."
But retailers have dismissed the union's claims and say smaller outlets can cope with the changes.
Speaking on the first day of the new pricing structure, Mark Bamforth, who runs Lytham St Anne's Post Office and Store in Lancashire, said everything was running smoothly.
He commented: "I think we're in a good position to explain the changes to consumers and feel that a good job has been done to communicate what is happening.
"I believe everybody was made aware of it and it shouldn't really be a problem."