Giannina, when i deal with my clients (all of them are from either US, EU or Australia) i make it clear upfront that i charge my fee in US dollars and also i make it very clear that i will be adding all the addon fees,charges,transaction cost to the final amount to be paid by my client...
If you don't add all the cost involved in receiving money from overseas clients you will find that quiet a bit of money is lost in transaction costs and you will eventually be getting an amount which is significantly less than what you expected/negotiated in the beginning...
-Anita CM
PS: If you are using Paypal to receive money from overseas clients then this tool will come very handy to you http://ppcalc.com/