Changes include switching to sea for outgoing mail and offering airmail and enhanced mail tracking.
From 6 July Guernsey will join Jersey and the Isle of Man in having the bulk of its outgoing mail taken by boat.
Guernsey Post CEO Steve Sheridan says the move will make the service more sustainable but it will add 24 hours to UK delivery times:
“All incoming mail already arrives reliably by sea.
"Moving outbound mail to the same model means customers will see minimal difference, while helping ensure a financially sustainable service for the future."
From early July, customers will be able to chose a tracked mail service so they can get updates on where their post is, and it is being offered at a price that is, in the majority of cases, cheaper than standard mail.
For urgent items, Steve Sheridan says Guernsey Post will begin a three month trial agreement with Aurigny, for next day delivery on mail and medical post up to 750 grams in weight:
"For customers with time-critical items, we are also introducing a next-day by air option, ensuring urgent deliveries remain supported whilst we test this service.”

Beau Sejour closed after legionella detected
Scorecard for people in Guernsey to monitor their alcohol intake
Plans to sell Guernsey rectory
Thousands of new species discovered in Guernsey’s freshwater habitats
Guernsey Post to end air deliveries
Channel Islands Pride: What's on where?
18 months before work can begin on resurfacing Alderney's runway
Guernsey man reflects on surviving 9/11 on the 25th anniversary