Jersey Post are blaming new price increases on higher costs from Royal Mail and the fall in value of the pound.
The cost of a local letter is going up by 1p and letters to the UK, Isle of Man and other Channel Islands by 3p.
The company say less post being sent, as well as the rising number of houses they deliver to are also responsible for the hikes.
The cost of some international letters has gone up, but those weighing up to 100g are now cheaper to send.
Tim Brown, Jersey Post’s Chief Executive, says the business tries "to minimise the financial impact of our business and industry change on customers" but they "face a number of major challenges".
Discounted rates at Christmas will still be available.
Changes come into effect from 11 April.