Householders in Caithness and north Sutherland are paying a single delivery fee to all addresses in their KW postcode area, which they shares with Kirkwall, despite wide variations in the actual cost of delivery.
As a result, members of Thurso Community Council are demanding a new prefix for the far north.
Last month, the Post Office confused customers in Thurso when it advised them to use Longhope in Orkney as their branch was closed for refurbishment.
Community council chairman Bob Earnshaw said the mistake highlighted the fact that many organisations consider Caithness to be an island when pricing delivery charges.
In 2011, Citizens Advice Scoland highlighted the problem when it revealed firms had been known to charge up to £110 for deliveries to the Highlands and Islands.
Two MPs have backed calls for retailers to make it clearer how much extra people will have to pay for deliveries to outlying areas.
Mr Earnshaw said: "This is an issue which has been bothering a lot of organisations across Caithness. The post office situation has brought this to a head, but there's also the serious situation of receiving parcels from the south where we are forced to pay island fees.
"We are planning to put something together to get our points over to address the situation.
"We want something which will identify Caithness and north Sutherland as part of the mainland and the KW postcode scrapped for the north Highlands."
KW covers Orkney, Caithness and the Sutherland areas of Bettyhill, Brora, Golspie, Helmsdale and Kinbrace. The rest of the Highlands is under IV, but the Western Isles has its own HS postcode.
Thurso Councillor Roger Saxon said: "Shetland, Isle of Man, Isle of Wight and the Channel Islands do not share their postcodes with the UK mainland but Orkney shares its postcode with us.
"I would back a campaign to create our own postcode, but I would go as far as campaigning to boycott the KW postcode entirely.
"The US can work their system with just six digits, but the UK system does not work for us here."
Royal Mail said it was unusual for an area to try to change its postcode but stated initials did not necessarily reflect geographical or administrative boundaries.
A spokeswoman said: "Royal Mail uses the street name, post town and postcode as its main routing tool for mail deliveries in a range of postcode areas, especially in rural areas.
"For example, it is clear that Wick on the mainland has a postcode using the KW1 sector and Orkney uses the KW15, 16 and 17 sectors.
"We are not aware of any complaints regarding the KW postcode being extended to the north Highlands.
"Royal Mail does not normally change postcodes unless there is a pressing operational reason to do so though, under the code of practice for postal addressing, there is a process for customers to request a change."