What about including a document with the item being returned following repair, that clearly states, “this is NOT a purchase, it was sent to us from (the UK (or wherever)) and is now being returned as a repaired item”?
You could clearly label the outside of the package with that statement as well. Might that help?
Well, that will not work. I sent an expensive guitar from Sweden to England for a warranty repair 2 years ago.
I filled in everything making sure that this was a repair and nothing else. I got a 'shipping receipt' from the Swedish customs and I thought it was all fine.
Then a few days later the English shop contacted me a wanted a huge amount of money since they had been billed by the British customs, as they had 'imported' the guitar. I of course refused since it would be the shop to prove to the customs that this is not an inport and that they should not be billed at all.
After some back and forth, and bringing in some more senior staff in the conversation, they of course fixed it without paying any VAT or taxes.
And when I got it back, the Swedish customs already had a record of me sending the guitar so nothing extra was needed to be done from my side.
The moral of the story is that YOU, the receiver of the goods need to prove to the customs that this should not be taxed. By default they will never trust the sender, anyone can write anything on the attached documentation and the customs authority is mainly restricted to their countrys citizens and organisations.
And I just got my 13 year old KPA back from Kemper after a swift repair, replacing a dead power supply. For a fair price and no worries about any custom issues.
Kemper even fixed all the dead leds for just the labour cost.