First my complements to Steve on his thorough explanation of the HARM codes for bicycles. Having both exported and imported material professionally and as a hobby I can tell you he is spot on with his advice. I would add only a few other tid bits.
Shipping by USPS is duty free up to a point. I have shipped bike frames and parts to Japan by USPS and believe me the recipient was charged duty. The buyer deemed it a better deal than any package carrier from a cost stand point. Issue is your limitation of size with the USPS.
Also, so far as importing. If you do your homework and choose and contract the carrier yourself and work with them to make sure all the proper documents are in place prior to shipment. You will make it easier on the seller and be in control of the costs and the shipment as a whole. If you leave it to the seller you can get a big surprise when/if the package arrives. Believe me from experience, sloppy documentation, erroneous pricing, intentional or unintentional errors can really cost you.
Also, dealing with different countries you will soon learn one size does not fit all. Some are business friendly, others are not. Some will work with you on a mistake other will seize the product and you will go through hell to get it back. Not the sellers problem, they have your money and they sent the material. I just finished a negotiation with a less than friendly business country in my professional life for material that was accidentally shipped to the wrong address. It took 10 months of filling out forms, getting legal advice and paying a fine to finally get the material returned. All this for a mistake in the address.
Ray Homiski
Elizabeth, NJ USA