So I bought a whistle from a large Seller in Ireland, I live in the US. I decided not to keep the Whistle. That said I went to mail the Whistle back to the Seller. I paid about $110 for the Whistle and to return the Whistle via USPS and on to An Post was $55! So OK I said I’ll go ahead and mail it. After a long time the Whistle arrives in Ireland, in Customs and the VAT will be $38! Well that’s it, I opted out just to have it sent back to me and not pay Customs…no charge to have it returned to me.
Well I had followed the Sellers instructions on returning the Whistle and they said you need to pay for the mailing back and there may be a Customs charge. However the Seller did not tell me how EXPENSIVE it would be to mail the item back to Ireland from the US and also how expensive Customs could be.
So if you decide you are going to buy an Item from Ireland and want to return it, be aware your experience could be like mine and it could be quite costly. Now that said someone else’s experience may not be like mine. Had I not opened the package I guess I might have been able to do a return to sender and avoided all this. Other Folk may not have had the same experience as me.
Not likely to ever buy anything from Ireland when returning it might be a possibility. I’ve learned my lesson…maybe should have keep it and sold it stateside 
From my experience, in North America there is a strong enough resale market for whistles that getting rid of a relatively new whistle in tip-top condition should be no problem. That way you pay nothing more and get most if not all of your money back. If using an online resale website, post enough comments describing an instrument’s condition, playability and sound and why it’s being sold. Give potential buyers some realistic confidence that the instrument will be what they expect. If it’s from any of the known instrument makers, you should not have difficulty. If it’s a no-name maker, far more difficult. 
That’s weird. If it was marked up on the customs form as Returned Goods, there should be no import charges or VAT because it’s not an import. It’s goods that originated in Ireland being returned to Ireland under EU mail order terms, and that attracts no duty.
But as RoberTunes says, at this stage it’s probably cheaper to sell it on to a whistler in the US.
As it turns out the Company I bought the whistle from says they will pay for the Customs Charge. That said I refused to pay for the Customs charge before I know the Sellers would pay that. So I’ll just get the Whistle back and likely sell it myself. It was weird that Customs put a value of 138 Euros when the value I had on the Customs form was $100. If I’m not mistaken I thought there was no Customs fees on an item that was valued at $100 
In the Benighted Kingdom the taxable value includes postage and packing and insurance. (And for good measure they add VAT onto the total as well )
Maybe that is how they got to that figure… Also I think there are umpty-one definitions of type of goods each with their own tax rate/threshold for each country of origin. It is not designed to be easy.
The customs form you fill out in the US should list a nominal value of $10, Country of Origin: Ireland. NCV (No Commercial Value) and mention “return to seller” if there is space.
Quite frankly, I have had a harder time returning goods to Canada.
Good luck!
Bob