So there has been a post about being able to add less then 10 euro or pond to you balance.
To be able to get 60 days of game time in world of warcraft I need 4 euros.
When I get in touch with support. They claim you are able to. But I get the error of the amount being to low to add to my account.
Their response is basically add more and buy other stuff in the store.
I would love to add more but I only have 4 euros that I am able to add.
So how do I need to progress from here
You can do that. When you go to the Shop page scroll down a tiny bit and select the CUSTOM option. Enter the amount you want to purchase to top up your Balance.
Here is the EU support article that goes over the limits of Bnet balance. The minimum purchase is 1 
EDIT - also remember to check if your region or country has any fees or taxes on internet services. You may need to account for that in Balance before purchasing game time. Last, be sure to purchase the 2 month game time card and not a Subscription. Subs are auto renewing and you can’t use Bnet balance for those.
yes today I have checked and it actually worked.
Yesterday I tried and it didn’t process