Business owners in Scotland's islands say they feel "forced" to plan around their unreliable postal service, which has left some worrying about their futures.
One electrician told BBC News she has had to buy spare equipment so she is not caught out by late mail, while a jeweller said he feels particular pressure to deliver goods on time for important milestones like birthdays and Christmases.
The watchdog Consumer Scotland is calling on Royal Mail to improve standards and make prices fairer for island communities - who can face surcharges of up to £50.
Royal Mail said serving island communities involves some of the "most complex delivery routes in the UK".
It said it depended on ferry and air services which can be cancelled at short notice because of severe weather.
The company also pointed to its £500m investment plan across the UK.
Amy Garrick-Wright, a self-employed electrician in Shetland, depends on Royal Mail to send away her test instruments for regular calibration.
But she said deliveries have become so unpredictable that she has been forced to buy a second test meter to keep her business running.
"If my main meter goes away for calibration and doesn't come back for a month, I can't be without it," she said. "So I've had to buy a second meter. It's that sort of planning that you're forced to have to do.
"I just don't assume it's going to be reliable in the first place."
She described one delivery - a specialist ladder - that "sat somewhere for a month" before she chased it up, and letters from HMRC arriving weeks after they were sent, delaying her tax registration.
Hospital letters, she said, sometimes arrived "almost to the day of the appointment".
The Consumer Scotland study found island communities in Orkney, Shetland and the Western Isles face unreliable deliveries, surcharges of between £10 and £50, and retailers who refuse to deliver at all.
The watchdog said postal services were a "critical economic lifeline" for the 94 inhabited islands whose economies contributed around £3bn to Scotland's economy in 2023.
But it added fragile delivery systems were holding back small businesses, including heritage producers such as Harris Tweed and Fair Isle knitwear.
In Orkney, jeweller Martin Fleet said Royal Mail had been "a bit of a lifeline" to his business for more than 30 years, but he now wanted guarantees about the future, including for the airmail service that has carried island post since the 1930s.
"Without that service, we simply couldn't operate our business. It's as simple as that," he said.
"We have to make sure that Royal Mail don't forget about us in Orkney, Shetland and the Western Isles."
Orcadian jeweller Martin Fleet describes Royal Mail as a "lifeline"
He said the pressures on the firm - Sheila Fleet Jewellery - were sharpest at peak times.
"Birthdays, anniversaries and Christmas in particular," he said. "You can't really move Christmas, so you've got to make sure you make those deadlines."
Both business owners praised local postal workers, putting the blame on the wider system.
"That's not the fault of the postal workers, who work extremely hard," Garrick-Wright said. "But structurally there's huge issues that aren't being addressed."
Consumer Scotland's Tracey Reilly said islanders were "very realistic" about what to expect but were being let down by gaps in regulation.
She said the watchdog had heard of hospital letters arriving after appointments had taken place, and prescriptions being delayed.
"Every area in Scotland has a postcode-based delivery target, and those are set and enforced by Ofcom," she said.
"But the island communities are actually exempt from those targets.
"We'd like to see them brought within the overall regime.
"It doesn't have to be the same target but we want some guaranteed minimum level of service, because we think that's only fair."
Consumer Scotland's research found 18% of people across Scotland had encountered delivery surcharges, but that rose to 75% of people in the Highlands & Islands.
"For consumers, it can really increase the cost of living - sometimes the cost of delivery is actually more than it costs to purchase the item," Riley said.
The report makes eight recommendations including transparent and potentially capped surcharges, clearer delivery information for shoppers, and minimum service standards for island postcode areas.
Possible fourth penalty
The calls come as Royal Mail faces scrutiny over its performance across the UK.
Regulator Ofcom opened an investigation in June after the company missed its delivery targets for a fourth successive year, delivering 75.7% of first class mail on time against a target of 93%.
It also missed its local target in every one of the UK's 118 postcode areas.
Royal Mail, which was taken over by the Czech-owned EP Group last year, has been fined £37m over three years and could now face a fourth penalty.
The company has committed £500m to improving its service.
A statement from the company said: "When a flight or ferry is cancelled, mail remains within our network and is moved using the next available service, including alternative routes where possible.
"The length of any delay depends on when transport links resume and capacity becomes available.
"Cancellations can happen at short notice and may affect individual routes differently, which means it is not always possible to give customers advance notice of every disruption."
Responsibility for postal service are reserved to Westminster.
Postal Affairs Minister Blair McDougall said he knew that postal services had not met the expectations of customers in the islands "for quite some time".
He said: "Securing Royal Mail's future has been a priority, which is why last year we negotiated a wide range of commitments from the new owners, including that they cannot take money out of the business until performance improves and a commitment to remain the UK's USO provider.
"We have no plans to change the minimum requirements of the USO in legislation, and this government will continue to hold their performance to account."