From today, a long-standing tax-free allowance from the United States for parcel contents valued at less than $800 (€685) being posted from Europe is ending.
The only exceptions are smaller letters, documents, and parcels containing gifts worth under $100 (€85).
The move means that anyone based in the US receiving a package via post above that value will have to pay a tariff.
An Post has said it is maintaining parcel services to the United States, despite the imposition of the new tax by the government in Washington.
What exactly are the changes?
Around three weeks ago, An Post and other postal services across the world were informed by the US administration that parcels containing goods must have taxes paid on them before they can enter the US and be delivered there.
Now goods shipped through the postal system to the US from Ireland over the €85 threshold face a 15% tariff, or tax.
For example, if you are in Ireland and are sending a present to someone in the US - maybe a lovely Aran jumper - which might cost €100, whoever you're sending that jumper to will have to pay the 15% tariff before they receive it, so €15.
While the changes will also impact businesses. If someone from the US is buying something from Ireland that is going to be sent via package, all items will now be subject to an extra 15% regardless of value.
This is a big issue here because over one million letters and parcels are sent to the US from Ireland every year.
Some countries' postal services have suspended parcel deliveries to the US until they have full clarity on the situation, but An Post is continuing its service.
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How will An Post's system work?
An Post says that it learned a lot from its experience with Brexit and EU customs and built systems to cope with the increased administration burden of sending packages to the UK brought about by Brexit.
As a result, the company said that it is well placed to quickly adapt to the latest US tariff, as it already has much of the infrastructure needed to deal with the change.
Managing Director of Mails and Parcels with An Post Garrett Bridgeman said: "99% of the gifts that go to United States are less than $100, so we don't see a big problem here.
"The big problem is a lot of business customers in Ireland sell online into the United States, so all of their products sold online now are subject to new tariffs.
"What we say to our business customers is make sure that you put onto your website that the products being sold are subject to the EU tariffs and that someone from An Post will be contacting that customer to collect the fee."
The way it will work in practice is that when a customer from the US buys something from an Irish website, the receiver will be contacted by An Post to notify them they have purchased an item that requires a duty to be paid.
An Post will collect the money from the customer - to be paid to the US administration - and will then send the item stateside.
Anyone in the US receiving packages from Ireland over the €85 threshold will get three reminders from An Post about the tariff and will have five days to pay.
If the tariff is not paid, the item does not leave Ireland and is returned to the sender.
Who's likely to be most affected by this change?
As Mr Bridgeman explained, 99% of the deliveries to the US will be under the €85 limit, so no tariff will be due.
But for businesses selling online into the US, a lot of their products will be over the limit, and parcel deliveries are the preferred option for many small companies doing just that.
One of those is Bohemian Football Club in Dublin, which is known for teaming up with charitable causes when designing its jerseys.
As a result, jersey sales are a huge part of the club's income - not just in Ireland but around the world.
The US makes up a significant chunk of Bohemians' online sales, but this new tariff means the club will be hit hard and until it has clarity on the new state of play, The Bohs Store will not be taking any more orders from US-based customers.
Chief Operating Officer with Bohemians Daniel Lambert said the US is "one of our top five countries" for sales and "probably represents about 15%, maybe slightly more of annual sales".
He said the online store is suspending all packages to the US "until people can understand what the implications are. Right now it doesn't seem clear what the impact may be".
He added the club is "really hoping for a bit of clarity in terms of the value".
"These packages are low in terms of value - it's not like it's thousands of dollars worth ... it's somebody who's ordering a football shirt.
"But if it's either too onerous in terms of customs charges or it's too onerous in terms of technical requirements at either end, that's where the logistics chain breaks down," he said.
What's the situation like in other countries?
This has very quickly become a global issue.
German, Swiss, Belgian, Austrian, and Danish postal services are among the European operators that have stopped sending packages to the US because of the new tariff.
Asian countries, such as Japan and South Korea, are also impacted and have suspended parcel deliveries to the US, as has Australia.
But it is worth noting that some private companies, such as Fedex and UPS, are still offering the service, but again the tariff will have to be paid by whoever receives the goods in the US.
Those suspending the service say it is not permanent and they're trying to figure out logistical solutions to this new administrative burden.
They could also be hoping that Washington might backtrack.
As has been seen to some degree with tariffs already, the Trump administration has been known to change its mind.
Also, there might be a backlash against this in the US when people can't get their international deliveries, which could potentially force the issue.
Why is the US removing this tax exemption on EU packages now?
As far as we know, no specific reason was given.
But it is reasonable to say it is part of President Donald Trump's overall America-first strategy that we've seen with the tariff announcements.
He's prioritising US business, and this is another way of doing that.
There is also a suggestion this move is designed to reduce the high level of Chinese products being sold via the EU into the US.
But in the short term it is going to hurt a lot of Irish people in the US who will have to pay a fair bit more to receive anything from Ireland by post that's worth more than €85.
