Thousands of staff at some major post offices went on strike on Christmas Eve in a row over the day's opening hours.
The majority of post offices were open until 1600 GMT on Friday.
Managers wanted nearly 300 branches to stay open all day, as they said two million people depended on them to access money before Christmas.
However, the Communication Workers Union said an early finish on Christmas Eve was a traditional right which its members have enjoyed since the 1970s.
Tube workers were also on strike on Christmas Eve over working conditions.
The Royal Mail said it managed to keep open many of its 294 main "Crown offices" until their scheduled close at 1600 GMT, with many post office managers stepping in for absent workers.
It later estimated that just 45 of the Crown offices were closed by the action.
Post Office chief executive David Mills said the call for strike action had been "unnecessary and unjustified".
He said customers needed the branches to be open "so that
they can obtain their benefit payments and have cash over the following four-day Christmas holiday".
Mr Mills said it was "entirely reasonable" to expect staff to work a full day, or most of it, on Christmas Eve as they were paid until the end of their scheduled duty.
"We are grateful to the vast majority of our staff who worked," said Mr Mills.
The Post Office's management said the Crown post offices faced competition from banks and building societies, which were staying open all day.
The Crown post offices lost about £70m ($125m) last year. The Post Office may close some of them as part of a shake up of its services.
Andy Furey, of the Communication Workers Union, (CWU) said post office managers were being mean and over-demanding.
Workers "deserve the time off to be with their loved ones and families," he told BBC Radio Four's Today programme.
The CWU said it was "absurd" to keep offices open beyond 1230 GMT, a closing time it said had been in place "for decades" on Christmas Eve.
The union said it had "exhausted every single avenue to avoid industrial action", but that the Post Office's "dogmatic stance" had left the union with no other option.
In other industrial action, Tube drivers on the Piccadilly line staged a 24-hour walkout in support of a colleague who was demoted after allegedly passing four red signals.
Extra bus services were laid on, but London Underground warned of delays, with trains to Heathrow airport running every eight minutes.
But there was some good news when the Rail Maritime and Transport Union called off a planned strike by signal workers on New Year's Eve after a deal was agreed
to end a dispute over jobs, pay and hours.