Prices on a small island are usually higher than on the mainland an hour away, and they rise further in peak season. The ferry schedule explains most of the difference.
Every item carries a freight cost
Food, fuel, building materials and stock for every shop arrive by boat, and each crossing has a cost that is divided across whatever it carries.
That cost is charged by volume and weight rather than by value, which means it falls disproportionately on cheap, bulky goods such as drinks and fresh produce.
The result is that inexpensive items show the largest percentage increase against mainland prices, while expensive small items barely differ.
Capacity is fixed and seasonal
A vessel has a set vehicle deck, and freight competes with passenger cars for the same space during exactly the weeks when both demand peaks.
Businesses therefore stock up before the season, which requires storage they may not have and ties up money for months.
Where storage is limited, shops run out during busy weeks and cannot restock until space is available, which is why island shortages appear at the height of summer.
Waste travels in the other direction
Rubbish, recycling and construction debris generally leave the island by the same route, and that movement also has to be paid for.
Island authorities carry disposal costs that mainland municipalities do not, and those costs are recovered through local charges that businesses pass on.
Peak season generates the most waste at precisely the point when transport capacity is most constrained, which compounds both problems.
Labour costs behave differently too
Seasonal staff often need housing, and on an island there is no possibility of commuting from a cheaper area nearby.
Accommodation that could be let to visitors at high rates is instead used for staff, which represents a real cost to the business even when no rent changes hands.
This is why island businesses frequently operate shorter seasons and longer hours within them, concentrating the year's revenue into the weeks when staff are present.
What this means for a visitor
Bringing bulky essentials from the mainland is common practice among regular visitors, and it is the reason ferry queues include so many loaded cars.
Prices generally fall with island size and with ferry frequency, so a large island with several daily crossings costs much less than a small one with three a week.
Booking vehicle space early matters more than booking a passenger ticket, since the deck fills long before the seats do in peak weeks.