OK, I had a nice long chat with one of their expert staff
We know the first part:
When you buy over £135, Thomann send you the parcel, UPS ask you to pay duty and tax, and charge you £11.50 for the paperwork
what if you don't like it?
Thomann sort out the paperwork, and you have to claim back the tax and duty with their help, you don't get the £11.50 back
here's the tricky bits:
what if you reject a £200 one, and ask for a £230 one as a replacement
well, it's treated as a return and a separate new purchase for customs purposes
most annoying case:
Item faulty after x months
you return it, they sort out paperwork so that there is no tax back into the EU, repair it and it comes back to you with no more tax to pay. But.... if they replace it, customs will sometimes class that as a new import, if there is any difference in spec. So you are back to reclaiming the tax on the returned guitar, and paying tax and duty and £11.50 on the replacement. So I'd recommend repair where possible.
Thomann deal with this stuff all the time with customers in many countries, it's just BAU, we are just not used to doing this, since we haven't been buying so much from further afield than the EU