Customs is really a hit or miss thing. They don't have a set formula for how much they charge, it really comes down to how the customs agent is feeling that day. They have full power to charge basically (within some parameters) whatever they want.
That being said, we at First Drop mark the value of everything SUPER low. Now, they can still charge whatever they want but usually charge a lot less.
The reason we don't mark things as "gifts" is because when being shipping from a company, that raises eyebrows and then things get flagged and your stuff gets stuck in customs for weeks/months.
We have never had a problem with anything getting stuck in customs (unless the customer supplied some incorrect shipping info) and guarantee all of our shipping, regardless of where in the world it's going.
Hope this helps,