Thanks,

I have already tried this way and even tried via several browsers, cleaned cache and cookies, but I’m still getting the issue with disabled button.
The sound expansion button becomes active if I add to cart any regular product and I’m even getting offer to add free sound expansion.
But when I remove regular product from cart the cart becomes empty.

Support proposed me to wait until monday for web developers or to purchase expansions from retailer (Sweetwater.com).

But retailer has unambiguous description of product.
Retailer offers to buy a sound expansion and no information about what I purchase exactly.

Toontrack EZkeys Sound Expansion
Piano Sound Library for Toontrack EZkeys – Choice of Grand Piano, Upright, Classic Electrics, Retro Electrics, Electric Grand, Pipe Organ, or Mellotoon.

I could not understand.
If I purchase this expansion from third party retailer what should they send to me (serial number, coupon code, etc.) and where or when should I make a choice of necessary expansion?
If it would some serial number should I register it in my products or should I use it as coupon code?
Retailer haven’t yet answer me for my question.

May be you can help me with this question?