I live in Ireland and so all of my packages are international and I have learned a few things over the years.
1. Start with the kinds of crafts that you decide to send. I do bookbinding but almost never send heavy crafts like that overseas, it is just too expensive. Always consider lighter materials BEFORE you start to craft.
2. Volume is also important, as Annchen said above, if it is say a throw cushion, could you agree with your partner to send the cover and s/he will fill it.
3. Volume is important so choose crafts that are very squashable! Use fabric items to pad and cushion anything less squashable, rather than adding weight with packaging.
4. From here at least, sending anything in a box (no matter how small) is almost always significantly more expensive. So envelopes are the way to go. Again you have to consider what you are making, most post offices jump, chew and mutilate all packages, no matter how well packaged. The last item I recieved, the postman actually threw it in the window, it bounced off the sink onto the floor, everything was in some way damaged, mostly repairable fortunately.
5. Padded, paper envelopes are not generally the best choice, you can buy thin plastic envelopes in large sizes, they have no padding, they are very light and incredibly tough. Use these! But always write on them with a ballpoint pen, not a sharpie or other felt tip, to make sure the address doesn't rub off. Plastic evelopes will also protect your precious items from rain..trust me, they will find rain somewhere along the line.
6. Talk to your partner about customs for their country. Europe has strict rules and adds duty to anything coming from outside of Europe. Some customs officers apply these rules more strictly than others. If for example you send a package to me and put a notional value of $100 on it. When it arrives to me, I will have to pay $33.50 to receive it!!!...so in every case to Europe (even if it kills you) always put NO COMMERCIAL VALUE on the customs declaration form. I had thought that gifts were exempt from this duty. but in my last tussle with customs officers, I learned that even gifts are not exempt from duty!
7. as was mentioned before, check with your partner about local regulations, for example I cannot send kinder eggs to the USA (and I have never done so
... honest injun ..lol) You can't send anything that might have bugs or seeds to Australia, like herbs.
8. Log onto your postal service online, weigh everything and work out the cheapest method of posting, it will save you an awful lot of money. And a personal tip..never pay for weight that you don't send. For example our postal costs go up in 1/2kg increments so if my crafts weigh in with about 1/4kg to go before I reach the next level, I will throw in extras, snacks etc to make up the weight. i figure if you pay for it, then use it!!
and keep on swapping with us folks from other countries, it is fabulous to receive things from a different country. It can be surprisingly reasonable to send from the USA to Europe if you know these tricks..