The Federation of Small Businesses estimates that 70% of the 4.8 million small firms in the UK rely on the Royal Mail for their post. It says that every postal strike costs its members, who have up to 20 staff, £300 each.
The FSB's Stephen Alambritas said that his members were struggling to find alternatives to the Royal Mail.
"Our members have tried to shift to other companies - they've found them expensive and actually a number of them have been turned away because the other service providers can't meet the demand," he told the BBC.
Views from postal strike picket lines
"We really want these postal workers and the Royal Mail to get back to work."
If the strikes continue for a prolonged period, contingency plans for delivery of hospital appointments and medical test results have been drawn up, MPs were told earlier this week.
And it has emerged that the Ministry of Defence may charter extra aircraft to ensure serving troops get their Christmas post.
Postal workers formed picket lines outside 37 mail centres in the UK.
Colin Elcome, on a picket line in Cardiff, said the workers had terms and conditions to protect, as well as an industry and a public service.
"A lot of this is payback time because we defeated them on privatisation. Rather than building the industry up and the public service up, it seems that some of them want to smash it into the ground," he said.
Royal Mail says union 'walked away'
There were about 20 people - including pickets and supporters - outside the Royal Mail's Mount Pleasant sorting office in London.
The mood was peaceful, with no sign of any tension or police presence, correspondents said.
About 20 people had gathered outside a sorting office in Leeds, while police watched about 30 workers staging a protest at Liverpool's main sorting office.
Delivery vans could be seen entering and leaving the Birmingham mail centre, despite the strike and the presence of about 40 workers demonstrating with banners.
The Conservatives said a complete change of culture was needed.
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