shipping rates preferences question

Hi, I've recently received my first order, that I've just shipped; the shipment, though, costed me more then what the buyer (from Germany) paid: even my preferences was correct (10,20 € for shipment under 50 gr from Italy to Germany), the buyer paid 7,70€ (the rate for national shipping of the same size, here in Italy). Why? Doesn't the system calculate if the buyer is in the same country or outside? Should I set a preferred an international shipping rate for every customer, to avoid to pay more than I should for any order? Something doesn't work correctly. Could you help me with this issue? Thanks