It is a shame this has had to happen and hopefully it will only be a temporary situation.
Unfortunately, for the volume of exports to the USA we feel that the additional time required on our part does not make sense and our labours are best employed elsewhere in the business.
Recently the Royal Mail has changed the way that parcels to the United States are handled, and also taken the opportunity to collect more data / enforce the requirement for tariff information for the rest of the world.

Most of the orders we send to the USA would be sent as a Standard Airmail Large letter, and if weighing 100g or under this would cost £4.30.
Prior to Brexit we would complete the customs declaration and the item would be posted. Post Brexit (pun intended) the information on the customs declaration would be captured by the Post Office counter. This is for the whole world, not just Europe and would add several minutes to every export order dispatch as the assistant keyed the information.

At the end of August, the Royal Mail and hence the Post Office introduced a new way of dealing with exports to the USA. The Post Office computer system (the infamous ‘Horizon’) now adds fees to the postage to the USA as follows.

The example is a 50g package with a value of £60.00

Duty Payable £9.60
Admin Fee £1.50
Guaranteed Landed Cost Fee £2.61
Therefore adding £13.71 to the £4.30

As far as I’m aware the way duty works everywhere else in the world is that it is paid by the receiver not the sender.
The admin fee becomes inevitable.
As far as I can tell the Guaranteed Landed Cost Fee is a charge to make sure the charges are accurate.

Part of our decision to suspend export to the USA is the additional admin burden this puts on ourselves. Yes we could probably get an estimate of the duty from a website, and yes, we would pass it on to our customer when we charge for the order, but the way things are changing, almost daily, this is a headache we don’t need.

The other part of it is that I’m fundamentally opposed to the UK collecting United States taxes on behalf of the USA.
I knew our Prime Minister had bent over but wasn’t aware of how far and which way.
I’m sorry for our valued American customers that didn’t vote for this and we will monitor the situation, restoring exports if this situation goes away.