Can't say much about cost, as that varies by shipping company/method...
But if I'm not mistaking the CN-22 customs label is all you need for normal mail. Additional forms may be required when using other (quicker) shipping methods, as they can aid in passing customs quicker.
Keep in mind that on labels/documents that can be read by anyone (like the CN-22 label) you want to put a vague and or discouraging item description to prevent people from stealing your package, but don't lie, customs officers don't like that
![Headphone Smile :) :)](https://cdn.head-fi.org/e/headfi/smily_headphones1.gif)
Something like "Defective Electronics" would work for the X3. On the forms/documents that only officials are allowed to read, put the truth, the whole truth and nothing but the truth, as not doing so can lead to your package being in customs for a very long time or even being confiscated...
Another good thing to know is that insurance might not always be the best idea. Insurance might give you peace of mind, but it's also like putting a great big neon sign on your package that says "Hey! Look at this! This contains something that is of value to someone!!". It will increase both chances of being singled out by customs, as well as being stolen. Also, when you do send insured, read the fine print, there's usually some ifs & buts to it...
Also, request that Fiio puts "Repaired under warranty" or something along those lines on the documents when sending it back. It greatly reduces the risk of being charged customs fees. And, if Fiio needs to bill you for some reason, have them send the invoice separately.
Just some tips & tricks from my 10 years of experience with sending & receiving packages all over the planet