Let's talk about how customs & VAT work. The whole idea behind customs is to allow different countries to control the flow of goods in and out of their borders. They want to incentivize some kinds of trade and disincentivize other kinds of trade. And, of course, we all know what plain old taxes are for.
Customs duties are taxies imposed on goods when they are transported across international borders. These taxes are based on tariff codes, which correspond to the type of item being exported or imported. VAT, or value-added tax, is a tax that countries applied based on a percentage of the item's sale price.
To simplify: many times, when your Kickstarter backer in a foreign country imports your item, someone will have to pay for customs duties and/or VAT.
Now customs and VAT don't apply to everything. Many countries do not have VAT at all, so that often does not apply. Customs duties only apply if the imported good's value exceeds the importing country's "customs de minimis value." (A similar principle applies to VAT). In the USA, an item imported from a country with a value of less than $800 USD is duty-free. Beyond that, pay up!
Lastly, you might be saying "how do tax authorities know what an item is worth?" Simply put, you – the sender – tell them. The value you tell them is the declared value.