Consumers are set to be hit with hundreds of euro in extra tax and customs charges for online shopping this Christmas if there is a no-deal Brexit.
A UK crash-out could add nearly €100 to the cost of designer clothing and more than €100 to the cost of some electronics, as VAT and customs duties will have to be applied.
Shoppers doing their Christmas gift-buying online in November and December will have to factor in substantial new costs if the UK is no longer in the EU.
Lynda Slattery, the head of Revenue's Brexit Policy Unit, said that products bought from the UK will cost more than they do now, with extra duties and VAT applied.
"Consumers will need to pay these taxes and duties before they will be able to get the goods they purchase," she said.
Irish people are among the largest online shoppers in the world, spending in the region of €5bn annually.
An Post insisted it has put in place contingencies to avoid disruption to the delivery of parcels with some 14 million coming from the UK every year.
This includes arrangements with Royal Mail in the UK to ensure trucks carrying parcels arriving at ports will be "green-laned" with parcels checked on arrival at An Post mail centres in Portlaoise, Dublin and Athlone, rather than at the ports.
A €15m parcel automaton machine will help to speed up checks.
An Post managing director of mails and parcels Garrett Bridgeman said that larger online companies like Asos and Amazon were working on a delivery duty paid model, whereby they would incorporate the Irish VAT rate of 23pc into their display price.
But European Affairs Minister Helen McEntee said: "It's important that people know whether or not the company or the business they are buying from is doing that, and that's what we're trying to highlight."
At present, any item purchased from a non-EU country that costs over €22 (or €45 if it is a gift) is subject to VAT, with any item over €150 also subject to import duty.
Examples on Revenue's website for goods bought outside the EU show that a €173 pair of designer jeans is subject to customs duty and VAT that bring the total cost up to €266.
A digital camera costing €301 is not subject to customs duty but is subject to VAT and other handling charges that bring the cost to €420.
A no-deal Brexit will also mean EU consumer protection law will no longer apply to items purchased from the UK.
Shoppers are being urged to know where the company they are buying from is based, to read its terms and conditions on cancellations, exchanges or returns, and to find out what VAT and import charges will apply - and how these charges will be paid.