-snip- I did not know blockchain works like that. It's not like I have an address A with 1 BTC and I want to send 0.5BTC to address B, so after that address A will be left with 0.5BTC, what actually happens is that address A is left with 0BTC and 0.5BTC goes to fee
[/quote]A well-written single-address client will do that, most wallets will use a "change address" for the remaining 0.5BTC.

I guess that API works like coinb.in which requires you to manually add a 'change address', or it'll use the remaining as fee.