When trying to buy RoS from US Blizzard store on an EU account, you're automatically delivered to the EU store so you have to pay 40 EU.
So instead of buying from an outer CD key sites like G2A, Kinguin, etc. and taking a risk, you can simply gift RoS from the US blizzard store (it won't deliver you to the EU store) to your email (not your battle.net account email) and use the code on your battle.net account.
I tried it and it worked