Royal Mail is considering scrapping daily delivery flights to Jersey and the Isle of Man, and delivering to the islands by ferry instead.
The company is running a consultation, which it says would "simplify and update" its operation, which has seen the number of letters sent to the islands declining over recent years.
It would mean it would take at least two days for parcels and letters to reach the Crown Dependencies from the UK mainland.
Letters and parcels sent between the Channel Islands currently go via a sorting office in the UK.
While Royal Mail says it currently has no plans to scrap Guernsey's mail plane, the company says "it is possible that we may do so in future".
If approved, the proposed changes to Royal Mail's service requirements would allow them to deliver all of Guernsey's mail by ferry "without future consultation".
The island is home to two of the UK's biggest online card retailers - Moonpig and Funky Pigeon.
In its consultation, Royal Mail explained the change: "Guernsey, Jersey and the Isle of Man are British Crown Dependencies and are not part of the UK.
"As such, they are not covered by the domestic requirements under Ofcom’s Designated Universal Service Condition, which includes providing a next-day delivery service to every address across the UK.
"We are therefore proposing to change the definition of ‘Due Date’ to these territories."
Morten Alexanderson who runs the Jersey-based e-commerce company Onogo said this could be devastating for his business.
"We've been using this route for 15 years now and that's what we've built the business on, it was started locally.
"So for it to be taken away at very very short notice will be potentially devastating.
"Ultimately in a nutshell, what feels wrong to me is that they're degrading the service, but they're planning to charge the same amount of money for it."
Royal Mail said it keeps its operations under regular review: "Royal Mail, Jersey Post and Isle of Man Post Office are exploring the implications of changing from a flight to a ferry the way items are moved between the UK and Jersey and the Isle of Man.
"This is in the context of rapidly declining letter volumes and would provide an opportunity to jointly reduce costs while demonstrating our commitment to minimising our impact on the environment.
"This is also in line with Royal Mail’s environmental plan which looks to reduce reliance on flights as the company works towards its 2040 net zero target."
Royal Mail is bound by Ofcom to deliver 1st-class items the next day, but it now wants to treat the islands as an 'overseas' destination.
Islanders and stakeholders have until Friday 2 June to have their say on the plans.
Royal Mail says its new arrangements will come into effect from Sunday 3 August.
You can find out more about the consultation and how to respond here.