First of all, I'm swiss
About the customs, as far as I know it works like this (only valid when going to Germany, other countries might have other laws).
Let's assume you bought something in Switzerland, like an expensive 5D MK3. In that case you would have to pay 21% taxes when going into Germany (other countries have other taxes), but you can get the 8% swiss taxes back.
If you don't pay them and they beleive you bought this camera in Switzerland, then you need to prove them that you bought it somewhere else. You can prove this by sending them a copy of the original receipt, which often carries either your name/creditcard details/serialnumber on it. Now if you can't bring this up (as you did not truly bought it outside of Switzerland) you have to pay the taxes, plus usually the same ammount again as punishment.
By sending stuff per mail, it's the receiving country which adds the tax. So in that case it depends on which country you send it to.
A new 5D MK3 body is currently around ~2700 EUR, depending on exchange rate.