Amazon has told its marketplace sellers that parcels going from GB to Northern Ireland will need a customs declaration from April.
Commercial goods going from GB to NI already require declarations but most parcels are covered by a three month exemption.
The government has not been clear about what will happen when that ends.
Amazon has therefore moved to advise its sellers on how to prepare for customs processes.
Northern Ireland is continuing to enforce EU customs rules at its ports.
This is as a result of its specific Brexit deal, known as the Northern Ireland Protocol.
That means there are new processes for bringing goods into Northern Ireland from the rest of the UK.
Amazon marketplace is an online shopping service that allows other retailers to sell their goods via the Amazon site.
Marketplace sellers can either get Amazon to look after their logistics, known as Fulfilled by Amazon (FBA), or post the parcels themselves known as Merchant Fulfilled Network (MFN).
Earlier this week Amazon published new guidance for its MFN sellers.
It says: "After 31 March MFN and SFP (Seller Fulfilled Prime) sellers will be required to complete customs declarations for all orders sent from the United Kingdom [sic] to Northern Ireland."
Amazon also raised the prospect that this could lead to increased delivery charges for Northern Ireland customers.
It told sellers: "For MFN non-Prime you can adjust your shipping fees for Northern Ireland if necessary.
"To do this go to Shipping Settlements and create a new delivery rule for Northern Ireland on any relevant shipping templates."
Some retailers are already using customs processes to ship goods from GB to NI.
When the government announced the three month grace period in December it said the move recognised "the unique circumstances of Northern Ireland, the impacts of any disruption to parcel movements in the context of the Covid-19 pandemic and specific challenges for operators moving express consignments."
It added that when it came to the long term arrangements: "Our priority is to have a pragmatic approach that allows us to comply with the [Northern Ireland] Protocol without causing undue disruption to businesses and citizens.
"HMRC is engaging with operators to finalise arrangements."