Analysis of more than 3000 complaints also shows some consumers in remote areas wait more than a month longer for parcels to arrive than the rest of the country.
Citizens Advice Scotland (CAS), which carried out the research, blames a "postcode penalty" that sees island communities typically having to pay a 500% mark-up on standard delivery of goods online.
Couriers tell customers that deliveries to the Highlands and Islands can take up to seven days longer than for the rest of the UK. But CAS warns the actual delay can be anything up to 35 days, causing a headache for those sending presents to arrive before Christmas Day.
CAS said it has uncovered enough evidence for the Office of Fair Trading to investigate courier delivery charges.
Susan McPhee, CAS head of policy, said: "Many Scots are being routinely ripped off by unfair and unjustifiable delivery charges just because of where they live. That is clearly and deeply unfair, and it cannot continue un-challenged.
"We call on online companies to examine their delivery policies and make sure they are not applying this unfair postcode penalty on Scotland."
The organisation found about a million Scots face surcharges, late delivery or are refused delivery altogether when they try to buy goods online. At least 75% of delivery surcharges made by retailers applied to consumers north of the Border. That rose to 82% on Scottish islands.
More than half of the 534 retailers investigated could not deliver to any Scottish island.
Citizens Advice said it means 100,000 people living on Scottish islands are left with less choice in where and how they shop. Around 14% of companies refused to deliver to parts of the Highlands and Islands.
It found the PA postcode area covering Argyll and Bute is one of the most shunned areas by delivery firms, despite some communities being less than an hour's drive from Glasgow.
The organisation said 335 retailers (63%) of those looked at charged extra for delivery to certain parts of the UK. And 72% of those surcharges applied to consumers in Scotland.
It also found 39% of retailers took an extra three or more days to deliver to consumers in certain parts of Scotland, while 69% did not offer delivery by Royal Mail despite the postal service's delivery costs being the same anywhere in the UK. .
Online auction site eBay came under most fire in the report with more than 299 consumers describing it as problematic.
Citizens Advice said a glitch in the firm's computer system had viewed the postcode for the Inverness area as offshore.
Mattresses World appears to deliver to the Western Isles, but not to Pitlochry in Perthshire –245 miles closer to its distribution centre
Amazon came out far ahead when consumers were asked to name a good delivery service.
CAS has warned retailers that delivery charges should be "based on costs incurred and not on arbitrary postcodes".
It said retailers should offer delivery by Royal Mail wherever possible and "work to ensure fair and equitable delivery costs across the UK".
David Martin, head of policy at the Scottish Retail Consortium, said: "Extra charges will apply to more remote areas where the cost of delivery is higher. What we think is fair is if retailers are fully transparent about their costs.
"We have always been clear that transparency is key, so consumers will always know before they make the final payment what the costs are."