A UK-wide strike by counter staff closed only a handful of larger post offices, the Royal Mail has said.
The strike will run from 0600 BST to noon
Staff at the 85 so-called "crown" or main post offices were on strike from 6am to noon in a row over the transfer of services to WH Smith stores.
A Royal Mail spokesman said about 13 of the crown post offices were closed, while the smaller or "sub" post offices were unaffected by the dispute.
The main postal union fears the switch to WH Smith will cost 1,500 jobs.
The Communication Workers' Union (CWU) also says that those workers who do switch over to WH Smith will face a pay cut and reduced pension provision.
"Members have been left with no alternative but to take strike action as management is steam-rolling ahead with closures of crown post offices across the UK," said CWU national officer Andy Furey.
The Royal Mail first announced the WH Smith plans last year.
Under the proposals, the crown post offices will be replaced a post office counter at their nearest branch of the retailer.
Monday's strike is separate to last Friday's Royal Mail-wide walkout over pay and job loss fears.