I know that only the first call to oraclize_query is free but i don't know how to send the small fee requested. Can you give a code sample ?
Marco from Oraclize here.
The fee is taken automatically from the smart contract balance when you call oraclize_query. There are two possible cases then:
You can see an example here
If you want to know exactly how much the call is going to cost you:
uint oraclizeFee = oraclize_getPrice(datasource)
where you specify the datasource from the list that you can find on the docs
If you have any more questions, feel free to ask also on our gitter channel