Was the option advertised when you bought the unit?
For completeness sake, I was not aware of this option at buy time. But still, it is disappointing.
Moreover, I'd buy it again. As
nctnico pointed out, I don't need any option
The binding contract question was just out of curiosity. I'm not going to sue anyone. I believe it was an honest mistake.
They sent the initial invoice (no option included), I paid it. The shipped the product.
The next day, I received a new message saying that the instrument has the option installed, and a
new invoice for that option for zero euro.
So yeah, maybe you should ask for a revised invoice. Particularly if it is going to affect your taxes.
No need since I don't get a deduction for this and the VAT is payed in this case by the vendor.