From what I understood I can use Apple Account balance on apple.com to buy products in available countries (mine included), but during checkout I don’t see it affecting the total amount to be paid. Any idea?
You can use your apple account balance to buy anything at the Apple store, brick and mortar and online.
However; their online store is a mess when it comes to using your Account Balance. I tried pre-ordering an iPhone and missed my pick up window on launch day due to the website bugs.
I ended up paying for my instore pickup with credit card; then when I went onsite to pick it up I asked the genius person to refund and rebuy the phone with my apple account balance. This worked for me when it failed miserably online.