you won't be able to do this with an international shipment. within the EU it may be different some how... i'm not sure what the cross-over is on stuff like that (from what i can tell, all "the post"s are still 'nationalized' and not supranational yet). like, you couldn't print postage in Spain and drive to France and mail it..., right? Or vise versa...
i do know for sure, only USPS approved stamps/meters/etc are valid for shipments sent from the USA. individual purchasers of stamps trade in local currencies, not the international postal standard. the local office wouldn't know how to process stamps bought outside their system (even though within the postal system they are exchangeable).
i think its called an 'international postal unit' but i'm not entirely sure. the various post offices throughout the world trade in those. but, regular citizens can't buy these, they purchase stamps/etc in their own local currency. which translates into a IPU, or whatever, but only within that international postal union system. its irrelevant outside of this.
so, in short, no you can't do this... but, again, inter-EU may be different but i have no clue what the postal rules are there for that stuff.