The research, published by Citizens Advice Scotland (CAS), showed islanders were typically paying £19 more for postage while they could wait up to 35 extra days for a delivery.
At least 75% of delivery surcharges made by retailers examined applied to consumers north of the Border. That rose to 82% on Scottish islands.
The study also revealed that up to 16% of retailers refuse to deliver to certain areas of Scotland.
Some 55% of 534 companies looked at could not deliver to any Scottish island – which meant 100,000 people living in these places were left with less choice in where and how they shop, CAS said. Around 14% of companies refused to deliver to the whole of the Highlands and Islands.
Annie McGovern, policy manager at watchdog Consumer Focus Scotland, said: "Citizens Advice Scotland's report provides yet more evidence of the delivery disparities that exists for consumers living in rural areas of Scotland.
"We recently held a major summit with retailers and delivery operators in the Highlands on creating 21st century delivery services. We are encouraged that a number of big organisations took a seat at the summit and have given a commitment to working with us CAS, the Office of Fair Trading, Highland Council Trading Standards and the Scottish Government to make services better."